All Xray diffs fit the entire range of touring cars back to the first (T1). You will need to change belts, but this is possible too because Xray has a range in one tooth increments for both front and rear. If you really want you can create a car with a mix of small diffs and large center pulleys just so it makes life easier when chasing a low FDR for stock racing.
Heck, if you really want, you can upgrade the old T1 to the latest and greatest from the T4, including driveshafts, suspension, etc. Keep just the main chassis, bulkheads, and top deck!
Team Greasy Weasel
The best upgrade to any car is some driver skill.