1986 M30b28 suitable differential issue

[e21 lover]

Hello,
i'm from egypt, and i'm owning a 1982 316, now i've swapped a 1986 m30b28 of the 7 series in the car, there is a lot of bad sound i hear from the differential as all the mechanics in the neighbor hood told me that the differential is ruined, it's 3.91 e21 open differential, but it wasn't in a good condition in first place, now i want to know the best choice, whether to buy a healthy 3.64 open small case differential of the e21, or to install the whole rear trailing arms and the differential of an e30 with a braking discs instead of drums, but that would cost a hell lot of money, its fine if that would be better, i need to know the whole story of this issue cuz i didnt expect the engine would ruin a 3.91 diff in 3 months!!

[e21-Mark]

I have a rare 3:45 m30 differential that I will sell. You could either fit the complete unit or rebuild yours, with the new crown wheel & pinion?

I can ship if needed?