Spider gears of the diff may be plastic though.
Either way, I found this thread which would be of interest to you:
http://traxxas.com/forums/showthread...r-differential
http://traxxas.com/forums/showthread...he-Italian-Job
Picture in the second thread shows plastic teeth in the spider gears.
Someone there also upgraded to rc18t steel ball diff - so that's a good option later.