I 'acquired' a furniture dolly from my father-n-law when I moved my gunsafe. I think 2 peices of 3/4" plywood laminated together with 3" heavy duty casters should do the job just fine.

 

This thread gave me another idea.

How about just throwing some carpet down and bolting a 2x4 to the floor of your garage about a foot or two from the wall and just lean your doors against the wall?

Obviously, you lose the benefit of being able to roll your doors around, and it's kind of semi-permanent, but it's extremely cheap! I think I might have to try that...