I know nothing about the XXcrt diff's but I run a XXXS and a XXX4 and I have never had a problem with the diff's in either.
I was running a 14x2 in mine and it didn't seem to have the pick up or top end I was looking for. I threw an 11x1 in and it was alot better. I'll probably end up throwing a brushless system in before I'm done with it.