I have 2 open diffs. wanna trade ? Ill trade you 2 opens for a LSD.

And there are always e30s at cophers U pull with open diffs. An auto car will have a 4.10 a manual will have a 3 something. you can also get a good one out of a e30 318, the 4 cyl cars were in the high 3s.

If you use the diff out of a 5 or 6 series you will need to swap out the rear cover, I am pretty sure the mounting point is different.
Plus most of the ratios are for hiway crusing on the 5 series.