The speed sensor for the speedometer is in the dif housing - looks a bit like a CPS. I think other speed related devices and systems get information from this unit too. Start checking there but check out wires etc carefully first - no point in buying a unit for a damaged wire.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i