First a little background
1997 GMC 1500 Z-71. No lift, no big tires. 90k on the clock.

OK I give up, I can't figure out what the hell is wrong. I put my truck in 4 wheel drive for the first time in about a month. And when I engage it, it's fine, everything locks in, front tires get power. But after about a minute of being in 4 wheel drive I hear a hi-pitched whine sound, and when I let off the gas, I hear this terrible grinding, sounds like a rock tumbler. When I hit the gas the grinding goes away but the whine is still there.

Things I have checked.

Front diff fluid, drained and re-filled
front ring and pinion - looks fine, no major slop. No chipping or missing teeth.
Front u-joints - greased em up, even though they looked ok
CV boots look alright, no cracking or big holes.

WTF?? I just don't know enough about the IFS stuff to really know what else to do?