Is it possible that there is a little play in the rear pinion yoke and/or rear diff? If not there may be play in the area that the driveshaft mates to the transfer case. Just two ideas that may help track it down.
'92 YJ Sahara
1 1/4" shackle lift for now soon to be some sort of lift
31" Bridgestone Dueller A/Ts on AR-767 wheels
'67 Ford Galaxie 500 (passes everything but the gas station)
"What do you mean I can't make it, watch me!" = oops