This is a hybrid central differential gear inspired by the work Steamerzone has done for the "OpenRC almost all metal rugged driveline" (http://www.thingiverse.com/thing:162810). Print the gear ring in nylon (e.g. Taulman 618) and the inner part in ABS. Both parts slide together and can be further secured with a drop of CA (in my case the press fit alone was enough). Alternatively you can print both materials at the same time with a dual extruder machine. This combines the strength and flexibility of nylon for the gears with the stiffness of ABS for the central housing.