when these diffs go, you know it. It will make loud grinding or clicking sounds!

If the power just isnt getting to either the front/rear wheels right, then suspect the center diff. If it goes, the power wont get to the front or rear.
Also wanna make sure none of the driveshaft slug screws are loose (center shaft input cups)

if they are bad, buy the gears you need AND buy part # 81069
its a shim kit. Sometimes the input pinion shaft or even the ring gear's mounts have a bit of play. Shimming them with the parts in the shim kit will make them last alot longer.
Simillar issue happens with some of the XTE and 8e models. The shim kit has any/all shims needed to get everything meshed well.