I've got a couple options for you. A 4.10 open diff from a 525i (m20), or a 3.23 open diff from a '95 525i. Your car should have one of these - depending on the tranny type (auto is 4.10, manual is 3.23). Both are guaranteed to be in good working condition.
You can check http://www.vinesauto.com/diffratio.htm for some of the different BMW diff ratios...
Thanks!
Bret.