[Al Skierkiewicz]

Matt,
I had been waiting for a mech guy to answer but apparently there is no easy source for this item. You have probably concluded by now that a differential is a problem when used on a steering vehicle. Without some form of slippage in the mechanism, a turn will drag one wheel and stall the drive motor. I know if one GM team last year who fabricated their own and it was beautiful. However, two separate motors works just as well in our application and is easier to implement.
Hope this helps.