Have the opportunity to get a LSD rear gear set.

How easy is it to swap into a stock S4 base rear end? .. .Am I going to need any alignment tools?

Do I need to change any bushings?. . . You know, all that stuff that makes you say "F***"

 

You will for sure want to replace the differential mount as you will be replacing the entire differential. Its not a bad idea to have an alignment when you're done with the swap as you will be loosening the front nuts on the rear suffrage for its removal. The exhaust must come down so plan on any needed exhaust gaskets.

 

like Dave says the Y pipe needs to come off, and the rear subframe needs to get lowered for the diff to come out.
if the diff is out the diff mounts should be replaced, especially the front one. the competition stuff is cheaper, and gets rid of the rubber band feeling these things have

taking the subframe out, and moving the toe adjustment is optional, so alignment is optional, although a good alignment with less rear toe makes a huge difference. if you do pull the subframe out, you might as well change those bushings too