1981 is the right year for a T176, it is an upgrade medium duty transmission and I have two of them, they came as standard on 1981 CJ7s in this country. Check that it is the T176 (and not an SR4 or T4, pictures below) and it is probably worth fixing it, not expensive or difficult to do yourself though you will need a few tools you will not have in your tool box. If a T4 then I would suggest replacing it with something else and adjusting the price accordingly.
The reverse issue can be several things. One I have encountered is a tendency for the locator pins (Item 6 on the diagram) that guide the gear stick to fall out / wear. You can check this by removing the rubber boot over the gearbox (half a dozen screws) and having a look at the pins and the play in the gearstick around those pins. Mine fell out and are now filed down old bolts held in with metal filler / epoxy which was carried out with the transmission in situ in a few hours of bodging. At least I now have access to all the gears. It is possible to buy replacement oversize pins for $10 that you can wire down to prevent them dropping out but I saved at least $4 plus shipping of $40.
Others report problems with worn forks and wear in the slot in the gear stick around the locator pins. If it was just gear stick wear I would vuy a second hand stick.
Worn forks would need the transmission extracting, which is a few hours of work under the vehicle and some heavy lifting. However you could rebuild the trans at home in a day or two and the parts are reaily available and not exorbitant. At the same time you could service / inspect the clutch, transfer case and U joints.
I woud not let a fault like that put me off buying, I could get it much cheaper and price in a gearbox rebuild which would give 10 years trouble free service.
BagusJeep lives in Bali.
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE