Hi, all. I've had a fairly benign vibration in my XJ since I did a 3" lift (that included slightly longer rear shackles to get to the right height). I finally decided to install a pair of shims I had laying around (3 degree shims, IIRC), and the vibration is gone, except for under hard(ish) acceleration, when it shakes the whole vehicle, though not severely... just enough to feel like a vibrating massage chair.
If I accelerate slowly, or am just driving along, there are no vibes at all.
I measured my pinion and output shaft angles, and the pinion shaft is pointing down 2 degrees more than the tranny output shaft. I figured this would be about right, assuming the pinion shaft raises a bit under hard acceleration, as the leaf springs wind up (presumably bringing the two angles to effective parity). But I never noticed this kind of vibration under acceleration before, when the angles were worse.
I am not running a SYE, just the stock shaft, and have new U-joints installed. There doesn't seem to be any play anywhere in the system.
If I'm calculating correctly, the original (non-shimmed) configuration would have the pinion angle 1 degree too high to start with, rather than 2 degrees too low, like it is now.
So... I'm interested in your best guess for my best option:
1) Find a 1.5 degree shim
2) Do a 1" xfer case drop (hate to do that because of ground clearance reduction... I've already got a bunch of dings on the crossmember)
3) Pull three spark plug wires so I can't accelerate hard any more