If I remember right, you had 6 inch diameter wheels. 18.75*2 (for 2 motors) = 37.5 lbs of force. This divided by the radius of the wheel = 17.5 lbs of force.

You would have to determine if you can move your robot on caster type wheels with 17.5 pounds of force.

[Edit] Rubber wheels on concrete has a value of .02 when doing these type of rolling based force calculations with wheel bearings having a value of .001 to .003 which is normally discarded. Even with a very poor caster and calculating at a value of .2, you would need 15 lbs of force. If you had amazing bearings on a smooth flat surface, with good wheels, you would need as little as 1.5 lbs of force to move 75 lbs. [/edit]

I would suspect that about 1/2 of the weight of your robot is in the form of batteries. Actually, I would say it is probably closer to 2/3 of the weight of the robot but lets say 1/2 just for simplicity sake. This is about 32.5 lbs. You could shave off at least 16 lbs if not more if you went with the LiFePo4 batteries. This would take the load down to about 60lbs. I would suspect that you could probably take this down by more by using these batteries, but without knowing the weight/amps/draw of the robot, I don't know.

I think @Toymaker's robot is around 40 lbs if I remember correctly. He has no issue at all with these moving his robot. These motors have no issue at all moving my 25ish pound robot with one battery and dragging an inefficient ball caster. I still need to replace that with an omni-direction wheel. If the caster is good, and the mounting system of these motors are good, and the weight is dispersed so that more of it is on the caster, then I think you would be okay.