Try this place. www.diffsonline.com
. They happen to have a 3.91 LSD for sale in the used area. And they probably can answer any question you have about diffs.
1995 325I, window tint, K&N, debaffled airbox, BMW #104 wheels, Nighthawk low beams, smoked side markers, smoked corners, smoked taillights, M3 stlye heated mirrors, various interior bits, Bosal cat-back.