The real key, if using 4 wheel drive, is to have a pair of high traction wheels closest to the center of gravity, and a pair of lot traction wheels on the other side.
For example, you can turn down your skyway wheels on a lathe, one pair just until the first layer of plastic shows, and the other pair 1/8" or so beyond that. Then epoxy and screw a thin strip of incline conveyor belt material (very high friction) to the smaller pair. You should now have two pairs of wheels, about the same size, with vastly different amounts of grip. This solution was used by 992 when I was on it in 2002, and by 190 in 2003 (although we originally had conveyor belting on our front wheels, but it came off in competition). Both bots also had a very short length and a wide wheelbase, which helped.
On the other hand, 190's current robot is very long with a narrow wheelbase, and all our tires our pneumatic. Needless to say, turning won't be a major part of our strategy this year.