I would say double check the U-joints at the T-Case output and at the rear drive shaft at the Pinion. See if there's any play in them. Also jack the rear of jeep off the ground to take the weight of vehicle off the joints to check them. I had a issue like this and kept over looking the u-joints until I took te weight off them and found a cracked joint.
Also while checking the u-joints, check the rear pinion yoke, see if theres any play up or down or side to side... it should only rotate.
Next if not that I would say check the rear track bar bolts & shock bolts as well.