Robonauts have a great video showing off some of the points discussed here.
https://www.youtube.com/watch?v=CAJBC-DDL9w (2008)
They spoke about the drive train in 2009. The fundamental problem with steering all four wheels together is simply aligning the wheels so they all point the same direction to begin with. This is why their pods are designed the way they are, and why the 221 product is based upon their design.