If you really want a 2wd, you could use two small omni wheels or casters on the front (or rear) of your robot and drive like a normal tank drive. We did this in the past and it did work well, you just don't have as much pushing power as a 4wd would. However, it should be lighter than a 4wd too, and still be fairly maneuverable.
I would not use a car type steering on a robot because of the limited area in which the robot must operate. It could be done, but I think that it is more trouble than it is worth.