Does your blue diff look like this upgraded one from Duratrax?
http://www3.towerhobbies.com/cgi-bin...?&I=LXHUB2&P=Z
Is just the sleeve damaged or is there visible damage to the case? If it's just the sleeve then you may be okay. I got a nice sized rock stuck between my diff an chassis plate once and it put a nice dent in my alloy FG sleeve but the diff still works.