What's bad on the C6? The old three speeds are pretty easy/inexpensive to overhaul if you can find somebody familiar with them as there's no electronics and because they were made for so long parts are very plentiful.
It won't be as cheap as a swap for a used trans, but if I was going to pull a trans/transfer case/etc I would feel a lot better putting something fresh in it's place.
Hopefully you'll find somebody who's got a rebuilt unit laying around, but I'd see what it would cost to get done. I had a buddy in Portland get a gasser C6 rebuilt and installed for just under a grand, which I thought was very reasonable to do that job with good parts.
Good luck in your search