Broke down on the PA turnpike Christmas day. I have a 1995 Tahoe 5.7 4wd. Driving home from the inlaws on the turnpike , I started hearing a knocking under the truck. Sporadic at first then slipped into neutral. Long story short is I noticed that if I pulled the floor shifter for the transfer case back slightly, clunking would stop. It appears that the transfer case is slipping into neutral and back to 4H thus causing the knock.  If I held a little pressure on lever, it would cease. Let go and knocking started and then would slip back into neutral.  Fluid looks good and at proper levels.  Pulled over so as not to do more damage. Pulling off the tow truck bed in reverse and made a loud fast grind sound. Like it was just off the teeth of the gears. No reverse gear obviously. Any thoughts?