I know very little about rear ends.

My 71 350 SS has a 12 bolt rear, and I assume it is either not a posi, or that the posi unit is not working. When I get on it, I only leave one black mark on the pavement.

If it is a posi that is not working, how easily is it repaired?

If it is in fact a non posi, how difficult is it to convert to a posi?

I believe the ratio to 3.73, as the rear wheels turn approx 3 & 3/4 turns for each turn of the driveshaft.

http://pg.photos.yahoo.com/ph/bbaker33312/album?.dir=6357&.src=ph&store=&prodid=&.done=http%3a//photos.yahoo.com/ph//my_photos

If you put both rear wheels up off the ground and turn one the other should turn in the same direction for it to be a posi........... if the other wheel turn in the other direction it's a non-posi. The method of turn per wheel & turns of driveshaft is only good on posi rears.

They definitely turn different directions.

it's a one legger.

you should be able to get a posi diff, stock, eaton, auburn, moroso etc...... and have it installed in that housing and be off and running. I'm sure there are other particulars to having this done, but you shouldn't have to get a new rearend, just the 3rd member and whatever else needed to swap out the diff.

The only way to tellfor sure what you have is to remove the cover and look at ti, but I'd say that it is probably a standard differential by the way that you described it. Here is a picture of a 12 bolt Eaton posi from a '60s Chevy. http://community.webshots.com/photo/55410292/1055410461041712107hkgqKB

To install a posi you will have to measure the backlash with a dial indicator before you remove the original differential. That way you will know what to set it at when you install the posi. You will need a good shim set to adjust the backlash. You will also need bearings for the posi. I have instructions for doing this if you need them.