[sanddrag]

If you find the HPI Baja car on their site, there's a link to a PDF of the manual, which includes an exploded view. This differential is probably the most readily-available and lowest cost COTS thing I know of that isn't the size of something like one out of a riding lawn tractor. I've never examined this unit myself, but I would suspect it to be of high quality. It is in the final stage of a 1/5 scale off-road RC car, driven by a ~1-2HP engine, so I would suspect it could stand up to whatever you're doing if you design it right.

If you build your own, I imagine you wouldn't be hardening the gears. That's definitely something to consider, since most differentials (including the HPI I believe) have hardened gears, increasing their durability.