hey , anybody out there that has had a shop rebuild a rear, whats it usually run?  stock gears

Bairs redid my 77 differential. It was about $800, if I recall. Worked out great- no noises or leaks. Very happy with the communication during the process too. 

They don’t say anything about doing 81 on their site. I’d call them. 

First you need to understand that there is a difference between common rebuilt diff's and custom-built diff's. It depends on the condition of the diff to start, the power, trans, traction, and use will determine how long it will live.

Just comparing pricing from one place to another is a mistake. Educate yourself on how things are done and then you will have a much better handle on if something is cheap or overpriced. 

The 80-82 Dana diff's were not HD units, they were the end of the line diff to cut weight for MPG requirements. The cars were mostly automatics and slugs, for cruising they were fine but today with guys dropping in engines 2-3x the power level and running them hard, the 80-82 Dana is not the best option, but it is the cheapest in cost and lowest in strength. They had the same issues with bad axle hardeneing, clutch retainers that stick out and get chewed up, poorly match aluminum bearing caps, and smaller parts overall. Same design pinon DANA used in the 63-64 diff's that snapped in half with abuse.