http://www3.towerhobbies.com/cgi-bin...?&I=LXJYM2&P=7 and
http://www3.towerhobbies.com/cgi-bin...?&I=LXBTA4&P=7
But I just realized you have the 18SS version which should have already come with these. Just make sure to shim the diffs properly and you should be good to go.
There's a lot more info on the HPI MT2 here >
http://www.rcuniverse.com/forum/m_1505231/tm.htm