when the front hubs are locked, or permanently live, and the transfer case is in 2wd the result is the wheels are driving the
axles and differential gears.
this puts the load, which is quite small, on the coast side of the ring & pinion gears.
bearings are active, but without a load not much is gonna happen.
to Shawn, you could test for possible changes by removing the front driveshaft.
if you try that pull the plug on the transfer case motor. accidentally engaging 4wd with the front drive shaft removed is a guaranteed expensive crunching sound.
EDIT: talking about '98+