I created a thread asking questions about a shorty header's upgrade and someone suggested that i swap the rear end out and that doing that would boost performance the most.

I live in California and cannot legally modify my ECU so i am wondering if it's possible to upgrade the rear differential without a tune?
I would think not but i thought i should ask just in case there is a way.

I have a 2008 Ford Mustang Base v6 automatic, I would think a manual transmission would be more likely to...

Rear Diff upgrade & no tune - possible?