when in reverse you have torque pulling against the engine and tranny (thus the t-case also) just the opposite of the way it does in a forward pull. It could be the mounts as suggested that are letting the t-case come up and tap the bottom of the body tub. It also sounds right that it is doing it occasionally in as you kill the engine. However in forward gears it would be pulling the t-case away from that area if indeed it is hitting as I suspect it is. Check the motor mounts and tranny mounts for excess wear or breakage. I bet that will be the problem.