Mammoth Center Diff
I bought a complete front/rear diff off ebay for $25, then I bought just the spur gear for the diff from hobby people.
Mammoth center diff spur
Doing that gave me an extra set of ring/pinion gears (plus bearings and a diff case) for the front/rear diff and it cost me the same as the center diff new. My center diff is actually my old rear diff.
As for the XT2 having diff issues, I would have to guess you would have less problems due to the fact that it probably has a center diff and not a 2 speed. The XLB buggy (the big one) has the two speed as well, so it would have the same diff issues I'd think.
This is a site I go to for random bolts/screws/nuts/washer...
http://www.mcmaster.com/