Well, tanks essentially use differential steering
We could technically compromise with the tracks and wheels. How about using 6 wheels? (3 on each side). That'll still give us traction and avoid getting stuck over large bumps. Steering both the first pair and the last pair of wheels can give us awesome maneuverability too! I've seen some modern APC's designed like that. Oh btw guys, the robot doesnt have to go SO fast. Just less than walking speed is sufficient. Yes?
By the way, can you guys understand this stuff?
Notes on Tracked Vehilcle Steering
It got too complicated as I scrolled down =P