"I personally do not recommend any of the cheap alternative fixes for driveline vibes, such as TCase drops, shims without SYE, YJ yokes, etc. Those items are bandaides to remove vibes, but they do not address the real issue at hand. Vibes are a physical indication of UJoint binding. UJoints are cheap to fix. Vibes are often mentioned in the forums as a problem because they are annoying. The real problem is the stress put on the output shaft and bearing of the TCase when you lift and increase the operating angles of the driveline. The more you lift, the greater the operating angles, the greater the stress. Think of it this way, if you take a wood dowel rod, place it over your leg, and roll it back and forth over your leg, this is similar to the output shaft of the TCase at stock height. Your leg is the output bearing. Now, take that dowel rod, leave the left side at the same pressure while pushing down with the right hand to cause a bow in the rod, and roll it back and forth over your leg. The left side symbolizes the shaft in a fixed position inside the TCase, while the right side is pulled downward by the increased operating angles of the driveline. The greater the angle, the more downward pressure, the sooner the rod (shaft) will snap at your leg (output bearing). The purpose of a SYE/CV shaft is to shorten the output shaft and distribute the forces of the rotating shaft through two UJoints instead of one. If you take the dowel rod again, but shorten the right side by 2/3, it is much more difficult to bend it over your knee when applying the same pressure that snapped the longer rod. You use shims at the axle end to remove the stress on the axle end UJoint, and the remaining stress is distributed among the two joints at the TCase end. My father once got onto me as a small child when I was flipping the switch to the power locks constantly. He told me that items are designed with a certain lifespan. If the lock switch was designed to last for 1,000 switches before needing replaced, and I flipped the switch 100 times in 2 minutes, I decreased the expected life by 10%. Ever since I try to think of everything this way. This isn't an exact science, but I think the point can be made here. Just making up some numbers for example, if the stock driveline was setup to last 250,000 miles before the TCase needs an overhaul, and you lift 2" and leave the stock setup and drive 25,000 miles like this, you decreased the life of your TCase 50,000 miles. Like those cigarrette commercials, with every cigarrette, you decrease your life by 7-10 minutes. It's not a perfect equation, but it should make sense. Vibes are not the culprit, they are a reminder that something bigger is going on in your driveline. Eliminating vibes is a good thing, but don't do it just for the vibes.
2-3" of lift isn't going to kill your TCase overnight, especially when compared to 6" on a stock driveline, but if you get rid of the vibes using those cheap alternatives, just keep in mind your TCase may not last as long as stock. The brand of lift isn't going to matter, the year of your XJ doesn't matter, the amount of vibes doesn't matter, if you lift it without a SYE/CV shaft, life is shortened. It amazes me that the XJ was designed with a SYE/CV shaft in the front of the TCase, but was not done to the rear. It doesn't make sense that is wasn't done right straight from the factory, but this can be corrected."
That was a cut and paste from one of my many other posts across other forums, but in short, I don't recommend a TCase drop or cheap fixes, so if you decide to go that route, keep it for a short period of time. It doesn't matter if you have an 84 XJ or a 2001, vibes or no vibes, the more you lift it, the more the stress on the TCase. Oh, one more thing, TCase drops are a false sense of security and actually add more stress to the motor mounts.
