I'd bet on a binding "U" joint.....Sounds like the same kinda banging I got when trying to go over a bank that was too tall & steep - the drive shaft loaded up and banged a couple times, sounded like hammer blows till I got off the gas. Thought I was going to have to finish the trail in front wheel, but it actually didn't blow the joint. As Rob suggested, try it in 2wd to narrow down where it is.

97tj, Shaved Ford 9 rear, 4:10's, New 35x12.5 BFG KM2's to test!, Warn x8000i, ARB's, Quick Disco's, Hand Throttle, & it's my DD. smile