2000 Tahoe z71 drive shaft clatter

[Josiewhales]

Concerning my 2000 Tahoe Z71, I just started having an issue where at speeds between 20 and 55 it starts to clatter like there is a mud flap rubbing on the tire almost, but it appears to be coming from the transfer case or forward drive shaft. I have only been able to examine it briefly so far but all of the U joints are good. When the vehicle is placed in 4wd, either high, low, or even auto, the clatter will stop. Also should I be able to spin the forward drive shaft by hand? It will move freely one direction only making a of a grinding noise. The other way will engage and I can rock the vehicle backwards. I would appreciate any input y’all may have!