interesting question. I don't know of one, but on the trike we pulled out of a dumpster the rear drive is to a solid axle. Now that I think about it, I'm not sure if its fixed drive to one wheel & the other can 'overspeed' or if both wheels are on freewheels so you're always driving the inner wheel. The point: there's more than one way to skin a cat; you might be able to use typical bike parts rather than adapting a differential. Though if you do use a diff, I'm interested in seing your results!