I think you will be better off just having it rebuilt.
You never know what you are going to get when you buy used. An open 8.5" rear will probably be $350 - $500, and a G80 posi will be $500 - $750. IF you can even find an 8.5" with 3.42's locally.
I had mine rebuilt at Jeff's...
If you have the time, but not the money, (like me, with four boys, two in private school), you could always go to the u-pull-it in Davie and grab an open 7.5 and driveshaft out of any g-body for around $150. I wish I would have known, I just threw out a 7.5 at Jeff's shop a few months ago...