[MetalMan]

I think a clutch between the motor and each diff (two clutches total) would work.
The idea I have come up with is more or less a ball differential with no gear and one of the outdrives will be replaced by a part that mounts to the outrunner's shaft. One problem I can see is getting this slipper clutch-thingy able to handle that much torque - normal slipper clutches spin at higher RPMs so the torque is less, but here, the RPMs are lower so the torque is greater.

Then, a one-way could even be used after the front slipper clutch to give us rear-wheel braking only, allowing us to pitch the rear end in turns.

Also, it turns out that the shaft that came on my motor will work to mount the dogbone cups to with only slight modifications! And the 6mm shaft I have is about 0.0015in. larger in diameter than the shaft that came on the motor (doesn't fit).