Could be a couple of things. I'm a Ford tech so it may be a little different for Chevy.
I don't remember how they have their locking hubs for the 4wd set up but what it sounds to me is that its not disengaging completely or engaging intermittently.
I know on the Fords the locking mechanisms will do that. Also its possible the bearing is starting to go. Is there a whine out of it while driving? If so then that's the bearing going out.
If not my fist thing to go after would be that locking hub mechanism.