A simple idea really, on the one side of the differential frame - gear teeth. On the other side - chain sprocket teeth. Chain is a little more forgiving of shaft flexing than gears.
Anyways. Kudos to VEX for coming out with the differential kit.
While you are at it, think up a way to create a variable (or constant?) resistance brake for the gear in the differential that rotates to let the gears attached to the axles do their slipping.
Perhaps something that we tighten with a screw?
Perhaps a fan that has to move a small pocket of viscous fluid?
Perhaps a small electrical generator (a modification of one of the Vex motors) hooked up to a dummy load?
Something to convert the differential into a poor-man’s limited-slip differential…