Strange...

Well, the argument could be made that it is something from the transfer case forward, since it only does it in 4WD. However, You're likely in 4WD when you're really stressing the vehicle.

I'd suggest that you take it off-road, and leave it in 2WD for a little while, and see if the noise occurs. If it does, that points to the rear half of the powertrain. If it doesn't, that points to the front half.

Your description sounds like either a worn u-joint or a badly worn gear. Can I assume that you have replaced the driveshafts' u-joints, as well as the u-joints in the front half-shafts?

It also could be a piece of metal in one of your differentials that every once in a while gets picked up in the gear oil and carried up to a meshing ring and pinion or spider gear. I realize that you've had the axles worked on, but it might be worth your while taking off the covers of your differentials and see if anything drops out with the gear lube. Then I'd douche the inside down really good with some type of spray solvent (carb cleaner or brake cleaner) before putting the covers back on and filling them.

Granted, these are long-shot suggestions, but you're done just about all the things one would think of first.