Regarding using a standard but narrowed differential in the back: I thought of something similar but the Trike idea is such a good idea, and legally trikes are easy to register, and putting two wheels in the back would blow away the whole trike idea, plus increase rolling resistance.
I think the trike near the bottom of the first post in this thread is what a lot of us are talking about, just a little longer for a second rider:
http://ecomodder.com/forum/showthrea...80.html?t=1280