Found it!! I went to camp jeep over the weekend and i was talking to one the engineers (Jim) so he came out and took a look. Its the t-case linkage, the tension isnt as tight so its making a noise. he said its nothing to worry about, i could replace it but its so rusted it would most likely break. He mentioned that he's seen people put a spring on it just to give it some tension so it wont rattle, so i could try that.

Jim was very helpful and says he really enjoys talking to people about there jeeps, good or bad. He said he's been developing wrangelers for the past 9 years and took a big part in developing the rubicon.

Thanks to all who replied to help. and i'll get some picts of camp jeep up soon...

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.