[Steeveedee]

If the diff is bad, it will be obvious when you take the cover off. If you mean the outside is clean, that's one thing. I blew up a ring and pinion in my '66 Le Mans but managed to limp it home to replace it. It was clean on the outside, too! Someone before I bought it must have been in there, because one of the ring gear bolts came out (not sheared off) and got in the gears.