The only other thing I can think of from my experience (other than a bad u joint) is maybe a bad trans/ transfer case mount. On my Ford, again, I had a worn out tranny / transfer case mount, and the transfer case would torque around under load, and bang against the floorboard. By chance is there any play in the slip joint in your drive shafts?

Too bad you don't have a spare jeep sitting around so you could swap parts until you identify the problem.

Sean

99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs