http://www.bmwe34.net/e34main/upgrade/Recipe.htm
click above link, then scroll down the page for some recommended diff ratio "upgrades", though if you have a 4.10 right now, moving up 1 level would be a 4.27 which can be hard to find.
BTW- what car do you have?? (525/535/530 all use the same type diff, 540/M5 use a different type)....
I've got a couple different ratios available, both LSD and open. Let me know what you want to do and I'll see if I can help.
Thanks!
Bret