[BJC]

Your description should allow for turning about the 12" pneumatic wheels. Are you turning at all? Where is the majority of the weight on the robot? How are these SuperShifters connected to the drive wheels. I think we need more information before we can offer a solution.

Edit:
Drop offset Assumes a 6 or 8 wheel skid steer. Your problem doesn't correlate with most things that will stop a dropped 6wd from turning.

The first thing to do is to check your omni wheels and make sure that the small rollers can freely spin.