My coffee grinder does that also. Try smaller nuts.
-OR-
When the transfer case is becoming warn the gears do not match as they originally did. Be sure to change the 4 wheel setting when the truck is stopped and the clutch is fully pressed in to disengage the engine from the tranny. Check the fluid in the tranny and transfer case every 10,000 miles and check some of the old fluid for excess metal (magnet).Change the fluid every 75,000 miles for normal use.
4 wheel drive assemblies are in need of more care than 2 wheel drive even if seldom used.