It could be quite a few things, from something simple like half shaft u-joints to worn spider gears or pinion or carrier bearings. Does it do it in "Auto 4x4"? If not, you can pretty much eliminate it being in the transfer case, and I would think the spider gears and carrier bearings as well.
Check for slop in your pinion, check fluid level. Might be worth even changing your fluid and seeing if you find anything in the old stuff. Good luck