I get the feeling wider back wheels would make it worse, a wider wheelbase turns more easily (less skidding), so those would grip more, taking the front wheels with them. It would drive fwd/back fine though. Again more weight over the front alone will do it, as it happens T2 and Rango both drive around the front wheels as there is more weight on them. If you can't shuffle weight around though, for the sacrifice of a bit of overall grip, the easiest way is to make the back wheels grip less than the fronts.

edit: wait, two different wheel types? Robochallenge wheels will grip way less than the banebots.