If you go the GA route, you will need 2 of these.
http://www.rc-monster.com/proddetail...LM13010&cat=17
and 2 of these, besides the diffcases and bearings.
http://www3.towerhobbies.com/cgi-bin...&I=LXTKK9&P=ML
I am not sure the above link is the correct ones for the 3906 E-maxx, but it is the 3906 ones you need.