We designed a swerve/crab drive last summer and built it for this year. Our main goals were simplicity and modularity as we do not have a factory/CnC to help, everything is hand built. I have several more efficient and complicated CAD's and designs at home, I'll see if I can dig it up later.
If you are interested, you can view our current system here:
http://hamosad1657.net/inventor/Chassis.html