[Al Skierkiewicz]

What several teams have started doing is either buying or designing/building their own wheels that grab in one direction and slip in the others. (We called ours BUPOD but I don't remember what the letters stand for at the moment. They were essentially small wheels mounted at the outer circumference of a larger wheel and at right angles to the driven axis. When driving the smaller wheels didn't move but when turning they did.)
Another alternative is to only drive one set of wheels and use casters (shopping cart) wheels for the other set.
Third and most difficult is to design a drive pair where the motors rotate as a module with the wheel and therefore drive and steer.
Good Luck,
Only two more days to go!

BUPOD=Biased Uni Planar Omni Directional.
Biased meaning they roll in only one direction.