Well, you've all been remarkably unhelpful
, but for the benefit of future users who might have the same issue, I'll input what I've found out on my own:
The problem is rather obviously the transfer case. When I put the truck on stands and crawled under, the differential, U-joints, drive shaft and so on were all fine, but moving up the drive train, I discovered a shattered TF housing; large piece missing, guts hanging out.
To my further annoyance, the housing had been wrapped with BALING WIRE, and must have been like that when I bought the truck about a year ago. Since I know I haven't hit anything with it, obviously the previous owner did a quick patch and dumped the vehicle ... when I bought it from a used car lot, I didn't have an opportunity to get under it to check out such things.
CAVEAT EMPTOR, folks. This is going to cost me another thousand bucks or so to fix; or I may just part it out and get a REAL truck. Certainly it's been said before, but NEVER trust a used car lot ... check it out yourself ... EVERYWHERE, even underneath!