Can't help you with why that happened or how to fix it, but I can offer some help with your question of replacing or upgrading. So you recently bought your GTO (so did I)...what are your goals for it? Do you plan on leaving it as is or further modifying it? If you plan on racing it some you still might be ok as the a4's are less destructive to the rest of the car than m6's are but my question to you is: While you already have to fix or replace the one that is in there, why not just upgrade it? You may go through another rear end if you simply replace it so spending extra $$$ on an upgrade may make sense. If getting the funding to do that is a problem (nothing wrong with that at all...I would be in the same boat) then disregard my previous comment. Hope that helps.
Congrats on getting the goat.