My car is a 1991 S5 Turbo, I have an S4 tranny and I bought an S4 LSD, can I just slap it in or do I need some other parts. PS I currently have a 2 way locking diff in that I want to swap for the S4 diff.

Thanks

 

you need the cross member for the transmission you bought.

 

And that's all you need. Plug n play assuming you're using the stock TII drive shaft.

 

The S4 tranny and the 2 way diff is already in the car, the S4 stock diff is not.

I think I have the stock drive shaft - it mounted to the S4 tranny - Do I still need the cross member?

 

you're fine..good to go.
(If that trans was not in then you would have needed the cross member as the s4 and s5 members are different.so the s4 cross member wold not have fit on the S5 Trans.)