Safety is the single most important factor when undertaking any modification project. Any changes should be carefully studied to see how the modification will effect other components. The planed modification itself must be looked at closely to see if it and the components used are safe. If you are planning this type of project, carefully review this material and research other sources before you start your project.
Face it, older classic trucks generally ride and steer like -- well -- like trucks. Being reasonable people (not prone to excessive masochism in the form of kidney-busting rides or arm-breaking steering exercises), we look for the appropriate corrective solution.
If you take the time to engineer the modifications carefully, do the work well and use good parts, most modifications will work exactly like you want them to. Poor design, sloppy work, or incorrect installation, however, will find you doomed to unsettling vibrations, aggravating noises, and premature failure of parts.
One critical aspect that is particularly misunderstood, and a common source of trouble is basic driveline geometry. Simply put, this subject covers the angles at which each component of the driveline rests in relation to the next. It may sound somewhat confusing, but I think you'll see it is not all that tough to understand. Besides, the reward for reading on may take the form of a smoother and more reliable driveline.
The engineers who originally designed these trucks made certain that the driveline geometry was correct from the factory. The problem for us is that we often change the original geometry when we alter the ride height, relocate the rear axle from spring bottom to top, use lowering blocks, change or re-arch springs, and all the rest of the myriad techniques we employ to create that perfect suspension. The bottom line is this: If you change the suspension components or configuration, you should verify the geometry is still correct. The sooner you do this and correct any problem, the less expensive and time-consuming it will be.
To measure these angles, you'll need a tool called an inclinometer. Every driveshaft or chassis shop will have one or more of these, and anyone doing chassis work should own one as well. You can spend more for more accuracy, but most hardware stores will sell you a perfectly serviceable tool for around $15. I purchased mine from Eastwood, also your local Sears store has them.
To check this geometry, you'll need to have the truck sitting on a flat surface, wheels pointed straight ahead, and with the truck complete and carrying whatever weight it carries under most conditions.
Setting up or correcting driveline geometry will involve shimming transmission mounts, putting angled shims between the rear axle mount pad and the springs, or otherwise rotating the rear axle to achieve the correct angles.
To get right to the point, the angle formed between the transmission output shaft and the driveshaft should be equal but opposite the angle formed between the driveshaft and the pinion. In Fig. 1 (ABOVE), this is shown by ANGLE 1and ANGLE 2. You'll notice that the reason these angles are the same is because the transmission output shaft and the pinion share parallel to each other. In practical terms, this also means that the angle between any flat shop floor (whether or not it is level) and the output shaft and pinion shaft will be the same. 
Figure #1
Although not always obvious, proper driveline geometry is based on the fact that there must be at least one degree of static angle between the transmission tailshaft and driveshaft and the driveshaft and the pinion. This is required so that the U-joints will oscillate back and forth enough to push grease around inside the bearing cups. If no angle were maintained, the action of the roller bearings would eventually wipe the U-joint cross-shaft dry and destroy it. You may have seen the ribs on U-joint crosses. Called "brinneling", it indicates wear and lack of roller bearing rotation around the cross. This condition is terminal for the U-joint. Because of this pre-set joint angle, the bearing cup not only rotates in a circular motion, but oscillates from front to rear twice per driveshaft revolution .
You can check this measurement by placing your inclinometer atop the center of your driveshaft and observe the angle. Next, place the inclinometer on the transmission tailshaft with the flat surface of the inclinometer perpendicular to the output shaft. Use an old yoke to get a good flat surface to measure the angle. Record your reading. Subtract the driveshaft reading from the transmission reading. An angle greater than 1 degree will achieve proper bearing lubrication.
Now, repeat this procedure on the pinion. An angle of four to five degrees is usual for the transmission tailshaft and pinion. Your goal should be equal, but opposite angles. In order to adjust components to achieve the proper cancellation, shimming of the axle and transmission mounts maybe necessary.
Viewed in cross-section (Fig. 2), rotating counter-clockwise, the cross is closest to simple circular motion at 3 and 9 o'clock. Traveling from 3 to 12, each arm of the cross attached to the driveshaft accelerates toward the driveshaft as well as maintaining circular movement. It decelerates as it moves from 12 to 9 o'clock and back to center, accelerates from 9 to 6 toward the transmission, and finally decelerates from 6 to 3 while moving back to center. Distances A and B are the additional amount the cross arm travels twice per revolution of the driveshaft, and account for the increased speed of the cross as it oscillates front to back and back to front. The resulting path that the cross arms travel is seen as an ellipse when viewed from the end of the transmission shaft.  
decelerating twice per revolution. It also means that at the other end of the driveshaft the same action is taking place, and is the explanation of why U-joints are always used in pairs. You see, the constant speed, circular motion of the transmission output shaft is changed, through the elliptic path of the U-joint, to a constantly accelerating and decelerating circular motion in the driveshaft which must be translated back to a constant speed circular motion in the rear axle (by the opposite action of the other U-joint) to avoid surging or vibration.
The key to making all this happen without setting up destructive harmonics, or torsion (flexing) in the driveshaft, and/or hammering transmission and rear axle components is making sure that the U-joints are in equal but opposite positions at any point in the driveshaft rotation. When the U-joints are in phase (the same point on both U-joints reaches the same rotation position at the same time and the angles between the driveshaft and both the transmission output shaft and pinion shaft are equal, the acceleration / deceleration cycles tend to cancel out, resulting in smooth and quiet operation. This is known as cancellation, and is the whole objective.
When perfect cancellation is not achieved, the regular motion of the transmission output shaft is still translated into acceleration/deceleration cycles in the driveshaft but it is not translated back to regular motion at the second U-joint. In this case, the alternating cycles of acceleration /deceleration are first taken up in any free-play that exists in the driveline. The resulting noise and vibration you will likely experience under these conditions are the feel and sound of bearings being hammered to death. Although this action is most often first seen in the premature failure of U-joints, pinion and transmission output shaft bearings, gears and splines take a beating as well.
In the case where the free-play isn't absorbing all of the hammering, components not designed to flex begin to do so anyway. In many cases the driveshaft literally begins to twist and untwist (torsion). This is quite easily felt and heard by anyone inside the truck, and is obviously very destructive. The only prevention or cure is to correct the driveline geometry to achieve cancellation.
The one major variable affecting driveline geometry is the tendency for the pinion to rotate up under vehicle acceleration and down under deceleration. Although all trucks have some pinion movement, the more weight that is carried, torque applied, and built-in suspension flex is present, the more the pinion will rotate. To compensate for pinion rotation when pulling loads for long distances or under constant heavy acceleration you may want to rotate the pinion downward so that under normal driving conditions (whatever they are) the pinion will rotate into a position resulting in good cancellation.
Given that the condition of the suspension components and the amount of torque being applied will vary, all over the place from truck to truck, there is no hard and fast rule to how much pinion pre-load is correct. However, most chassis and suspension people I've run across believe that no more than three additional degrees of pinion pre-load are advisable. For high stress, momentary or short-term pinion rotation beyond this, a pinion snubber is a good way to limit rotation, prevent banging, and preserve cancellation. - CTS