Believe it or not, i used to have an RC truck that worked just like you're describing. I actually only realized how it worked after reading this (given I havn't played with it in years - I knew it went slower when stood up, but i never had thought about why).
It had the motors built into the rotating part with the wheels, as you've discussed. It was skid steer, and it would stand up or squat down - giving it the adjustable speed.
I've found one picture online. I'll see if i can find mine in the garage tomorrow to get better pictures. We moved last summer though, so I dunno how lucky i'll be.
- Jeff