[sanddrag]

If this turns out to be more trouble than it's worth (which I'm not saying it necessarily is) there is always the option of making a traditional fixed ring planetary, and having the output of that be your wheel axle. You'd have to take some precautions so that wheel impact doesn't happen or if it happens that it doesn't affect the gears. It would end up being a little bit long to space out the bearings (probably an inch of spacing between them will do), but you could have the wheel double back over the gearbox sort of like the MER rovers. You could have a deep dish wheel so the gearbox goes inside of it a bit.

I'm even thinking maybe a Chiaphua/Dewalt directly driving a wheel by modifying the output shaft and end of gearbox to give better support for holding a wheel. The only thing is though you wouldn't want to go any larger than about a 5" wheel straight on a Dewalt, but at least I think that is enough to clear it so the gearbox can go inside the wheel a little bit.

The thing with making planetaries is that I haven't been able to find a source of wide face internal gears. The largest I've found is 5/16 and it is brass.