Only thing I can think of is that either the ring gear or pinion gear have a problem...or more likely, splines on the pinion or yoke are shot. Dad's gone through a couple yokes...they can get loose and when they do, the splines are pretty much history.
Either way, I'd sure take the axles out, pull the pig and take a look-see.
Either way, I'd sure take the axles out, pull the pig and take a look-see.