I think I'm going crazy, bought a 4.88 to slip in my EP 83 and it wont fit, its like the ring gear is about 1/4 to 1/2 inch too big. I have been using the stock 3.90 that came in the GSL swap I put in a few years ago and I think its an 83 also , but this new diff will not bolt up. its like it comes up 1/4" too big, three of us have been twisting and turning and that thing will not go in. Everything I read says the 1st gens are interchangeable. anyone else have this problem?

 

they're interchangeable, but the axles are bigger on 84 and 85.... make sure you don't have the diff upside down