[GMKlenklen]

And if you feel like moving on to 4wd... I'd suggest four cims, two gripies, two omni-wheels. arranged like so:
front left = gripie
front right = omni
rear left = omni
rear right = gripie

or a mirror image...

this reduces scrub to a negligible amount, while keeping it hard to apply torque to the drive train in order to turn you around (a problem if you load both omni wheels in the front or back, or less noticeably on one side). the only draw back is that your robot may have a bit of a "pimpwalk" as it bounces around on the omni-wheels... or your heavy enough that it doesn't happen.