Sounds like a classic case of the mainshaft nut coming loose, easy to repair, costs nothing.

Under the vehicle, on the rear of the transfer case is an aluminium cover, this looks like a small bra, remove this and two shafts will be exposed.
On the upper shaft is a nut, with a 30mm spanner, tighten as tight as you can, punch the outer part of the nut into the keyway to lock it.
Movement in the shaft should be eliminated, and the noise should disappear, if not it could be worn selectors, damage to the gears, or baulk rings, or the main gearbox bearings worn.
Check oil levels in the gearbox, and transfer case while the vehicle is in the air, or better still, replace it and add an oil additive.