Could possibly be the clutches if you have a limited slip in the rear. You said the fluid was changed recently, did you have the condition prior to the fluid change?
Limited slips require slip additive.
Have you checked the splines inthe rear driveshaft? is there any play in them? When was the last time the splined were greased?
If all the above check out, there could be a strong possiblity the spider gears are worn, especially if it is a d35 in the rear.
2000 TJ 5.5 Long arm, lots of extras. R.I.P.
2005 LJ rubi 35's Clayton, lots of extras