Last year we wanted a very mobile robot and planned for a 2-wheel drive with casters. I introduced the idea of HDPE "glides" instead of casters for a couple of reasons. The rounded "skids" worked very well as they slid smoothly on the carpet as well as the ramp, and we avoided any resistance due to casters having to orient the correct way and/or fighting each other "directionally". They were also large enough to span over the grid in the ramp wire mesh, therefore eliminating the bouncing we would have seen from using casters. We also needed to raise and lower the back of our robot and used the pneumatic actuation system shown below:
http://us.f1f.yahoofs.com/bc/63e9492...58G8_AwWYPomxv
This arrangement worked great, and using the large pneumatic tires we also had a significant amount of traction.