1986 IROC auto (assuming 700r4)

First off, the shifts are ridiculously harsh. No matter the throttle position or speed, it just slams from first into second. I've checked the fluid, and it looks good. I'm assuming the previous owner put some sort of shift kit in it, but I'm not totally sure. I'm not one to really mess with a tranny, so would perfer to let someone else mess with it. What does it usually cost to have a tranny rebuilt (rough estimates are fine) and will a rebuild remove the shift kit? Should/can I have them install a TransGo in the process? Or would it be in my best interest to just get a new tranny and if so is it recommended to stick with the 700r4?

The U-Joint just blew out on my car. I'm assuming the harsh shifting is what caused it. I originally thought the rear-end was gone, but turns out it wasn't. So I ran out and got a rear-end from a 91 RS with 2.73's. My question is, can this rear end hold 3.42's and posi? I'd like to upgrade my rear end to a little beefier gears.