So what happend? did you call Kurt@URCHobby ??? or RedCat? Anyone Else?
Is the diff case cracked?? Only a little crack can cause the pinion to not engage to the ring that is what happend to mine I had to hit a tree at 40mph to crack it though Put it all back together and it acted similar to what you are saying. Check the case or the pinion shim could have changed????