I have a Ford 8.8 in the rear and Dana 30 up front of my '99. About two years ago I had them regeared to 4.56. Perfect for 33's and way I intended to drive my XJ. I know that regearing results in a higher driveshaft RPM so I had the driveshafts balanced and replaced the ujoints. A vibration came along at 75mph so I had both driveshafts checked and re-checked.


I've already changed everything else I can think of that would be making such a vibration-
Wheel Hub Assemblies (Timken).
Drive Axle U-Joints (Spicer)
Upper and Lower Ball Joints (Spicer HD)
Differential Fluid - 75w90 Royal Purple (Been about a year...)

 

Tire?

 

With 33s i'll assume you've lifted your xj. You don't mention whether you've done an SYE. in any case, check your pinion angles. Both of them.

 

When re geared to match stock engine RPMs you don't actually run the drive shaft faster, you just spin the axle shafts slower. The new gears compensate for the larger tires and spin them slower to account for the larger circumference so that you get the same ground speed.

I would second checking pinion angles, and also check the engine and transmission mounts. They get too soft/worn out and angles will bonkers under load.