My XJ vibrates above 50MPH and just gets progressively worse above that speed. I recently installed an Iron rock 5.5" lift with new leaf springs and long arms in the front. I also installed a hack and tap SYE kit with a new drive shaft with a double cardan joint at the transfer case end. I did not think I would have vibe issues with this setup but I do.

I took the new rear drive shaft out and the vibes went away so I had the new rear drive shaft balanced. This did not help. I checked the angles of the drive line, the results were: TC 86*, DS 72*, Diff 80*. Next I decided to install a 6* axle shim kit to get the diff more in line with the DS. This did not help much if any. Now my angles are: TC 86*, DS 72*, Diff 73*. This leaves a 14* angle on the double cardan joint. Is that too much? I thought double cardan joints were made for extreme angles? What else could be the problem?

The cross member for the lift has a lower position that you can use to do a TC drop, but I really don't want to do this because of the ground clearance I would lose. The instructions for the lift say that this should not be necessary.

What else could be wrong? I feel like if I had a regular DS without that double cardan joint, it wouldn't be a problem.