[Jeffy]

I see two things that should work well for you:
1. High traction wheels in the center. Other wheels on the outside. Using the "medium" traction wheels on the outsides will give you more traction.
2. Use high traction wheels for the center, and either the front or the back. Then, use the omni's or these "medium" traction wheels for the other set. This should make you a "rocker" drive train, but with one balanced, level potion instead of two "rocked" or "tipped" positions like a normal rocker drive train.

Best of luck
-Jeff