99 suburban - front-end whining

A couple of months ago a whining sound started coming from the front of the suburban. Reminded me of the sound my old Ford made when the fan clutch was going out.

After some tinkering, I found that if you switched the 4wd to auto the noise would go away. After switching back to 2 wd the sound would disappear for awhile then return. Any suggestions... it does seem to be getting worse as time goes by

noise which seems to be coming from front can be amplified from the rear; check pinion gear nut at rear differential and universal joints