One of the most commonly asked questions, and undoubtedly, the most annoying of problems, is driveshaft vibrations. I'll admit to having had my share of these problems.
Whenever a vehicle's ride height is increased through suspension modification, the driveshaft angle is also increased. This often results in unequal operating angles of the connecting U-joints. While they're still turning the same rotational speed, the joints can slow down and speed up at independent rates, which often results in vibration. While not textbook-correct, I've found that when a short shaft and excessive angles are encountered, it's often possible to alleviate most of the vibration by pointing the pinion yoke almost directly in line with the driveshaft. Actually, I like to have the pinion point about 1 1/2 degrees downward. It helps prevent some pinion "climb'' and provides for near-perfect alignment when under load. Sometimes it may be necessary to include a CV-type joint at the transfer-case end.
A common method to alleviate excessive U-joint angles is to lower the transfer case and turn the pinion yoke upward by inserting shims between the spring and axle pad. While this may work, shims have a nasty habit of breaking and falling out. Another problem is that they don't often allow for full spring center pin engagement to hold the axle in place. Over the years, I've made my own shims, built longer-headed center pins, and finally resorted to cutting free and rewelding the axle pads to the axle themselves. After doing the cut-and-reweld gig three different times on my Jeep due to suspension changes, I decided there had to be a better way.
The adjustable spring pads shown in the photo were simple to make and offer unlimited adjustment of pinion angle. Construction was pretty straightforward. I located a section of tubing just slightly smaller than the axle tube diameter. A section the length of the distance between the U-bolts was cut in two lengthwise, giving me two half-rounds. The two half-round pieces were placed over the axle tube and held with a C-clamp. Two pieces of heavy-wall, 5/8- inch-inner diameter tubing were welded to the opposite sides of the half-sections with a temporary spacer between them and a bolt for proper alignment. These were then removed from the axle, bolted back together, and the lower section of the two halves welded to a spring pad. The upper half section of tubing was drilled and tapped for a 1/4-inch Allen-head center screw.
Now, the adjustable spring pad can be bolted in place on the axle tube, the U-bolts and springs connected, and the complete rearend housing rotated to the selected pinion angle. Once you've established the correct pinion angle through trial and error, a dimple in the axle tube can be made to tighten the set screw into as an extra measure to prevent rotation.