In 1981 the housing and mount were all cast as one piece, looks like it's either the rubber mount or something else.

1981 diff. ----- Your soooo right. My bad. Still not that bad to change out however. Keep us updated on your findings. I'm intersted to know what you find.

|UPDATED|4/10/2016 6:29:11 PM (AZT)|/UPDATED|

'80-'82 cars had a different rear diff set-up than the earlier cars, but it still has the front mount that deteriorates, and causes edzachary what you describe. It's next to impossible to see the upper part of the mount to see what shape it is in. That mount takes 95% of the torque load from accelerating/decelerating, so it is subject to a lot of abuse....time to swap in a new one. Only problem with yours will be getting the old one out, and the new one in. The earlier cars have a bracket that bolts to the nose of the diff that can be removed to change the mount. The late cars have that particular "bracket" made onto the diff housing. You may need to loosen the two bolts on the "batwing" of the diff to allow enough movement to get the mount out/in.

|UPDATED|4/11/2016 9:51:30 AM (AZT)|/UPDATED|

You also say you have an 1/8 - 1/4 inch travel when you push/pull tire at top and bottom. When doing this look at your side yokes on the diff and see if they are moving in and out while doing this. You may have some excessive inside your diff.