We
almost used a differential on our bot, till we thought that it wouldnt have helped us that much, and it ended up that just putting a 4 wheel tank in the back with two wheel steering the front(not casters but steered by a globe motor) worked great, ended up in 3.8 s autonomous and easy and fun drivability during matches.
here's a pic, u can see the 4 wheel tank in the back with the two wheel steering in the front,(no only the battery in the back is plugged in the other two are for weights to simulate the arm weight).
http://www.chiefdelphi.com/forums/pi...&quiet=verbose