Ok, so I've been searching through the various boards and I haven't found a concrete answer to my problem.

I just completed an AC 3" lift on my '02 SE/SC Xterra this past weekend. The lift consisted of new UCA's, T-bars and a 3 leaf AAL for the back, with stock shackles.

Currently the front of the truck is sitting @ 36", and the rear is sitting @ 38 1/4 " (measured from the ground to the fender lip, inline with the center cap on the wheel).

It's Ass high, I know, but I'm waiting for the AAL to settle a bit. The front is cranked to about a 1/2" from the bumpstops.

My truck has about 95K miles on it, so I have a few ideas what is causing my problem, but I'm looking for some advice and possible confirmations on my speculations.

After I put the lift on, I went driving around for a bit to test things out, I noticed that I have a slight vibration between 20 and 25 mph. It goes away once past 25mph, and does not return at all. Speeds above and below are fine, all the way up to 75mph, no vibration is noticeable.

If I am accelerating, I can feel the vibration start @ about 20mph, and it goes away @ 25mph. If I let off the gas, the vibration goes away.

Now, this problem only appeared after the lift was completed. I had no problems before the lift.

My guess is that I have some kind of driveline vibration, and I'm trying to figure out what is causing it.

Some of the options I have been considering:

1) The driveshaft is now at a different angle then before the lift, therefore the U-joints are operating at a more extreme angle causing the vibe. I'm skeptical on this however, because I would have thought that a bad U-joint would cause a vibration @ any speed, and not just at a specific speed range, like an oscillation. Also, because I had no problems before the lift, I'm doubting that it's a U-joint, but I'm certainly not eliminating the possibility.

2) My driveshaft is out of balance. Skeptical on this one as well, because there was no problem before the lift, and I didn't go anywhere near the shaft when putting my AAL in the back.

3) Due to the change in angles, I may need shims for the rear axle to correct the change in angle that the DS enters the rear axle.

Thoughts?

I have a similar lift on the back of my X without issue, but #3 is the most viable option.

Get a cheap angle finder from Harbor Frieght or similar cheap tool store and chedk your pinion angle vs. the Tcase output angle. You're likely around 4-5 degrees at the Tcase. Check the difference at the pinion. They need to be close to the same angle. I have almost the exact same vibe in my Frontier right now after installing new rear springs, and I have about a 7 degree difference in the angles. I haven't shimmed it yet, but plan to as soon as I have the time.