Thanks to all who replied! Yeah, a friend of mine curbed my car hard,
and took out the entire rear suspension,effectively :( Anyway, I found
an '87 325isa in a junkyard with an intact rear suspension, and hence
my question (which this answers quite well. Of course, I just pulled
the rear suspension anyway and confirmed that it's a 3.73 LSD by
reading the tag on the back of the diff...)
Can't wait to fix my car and start driving it again!