[Al Skierkiewicz]

Blair,
I think you are getting close to understanding why crab systems have evolved to where they are now. With the system that Ken has suggested (which I think is exactly the solution you were looking for) the side loads and frictional loads remain the same as a traditional crab drive without the ability to change directions incrementally. With only the 90 degree motion, you are limiting yourself to a defensive move that may not accomplish what you want. A true crab drive needs to back away before "exiting stage right" in order to break the friction with the robot it is pushing against. The side loads of an instantaneous change are significantly higher than a motor driven steering system.