So I bought a used diff out of an S4 TII + it's got oil all over it. The oil is over the front part of the diff so I'm assuming that it's leaking from where the driveshaft connects. Is there a gasket for this? Or an O ring or something like that? I tried searching mazdatrix, but found nothing.

 

It's a seal. You need to unbolt the driveshaft flange, pry out the seal, and then slide the new one in.

See the parts catalog in the 2nd gen FAQ.

 

Thanks Aaron