I have a 4'X 9' chicken tractor where I keep my six hens. It has a 4' X 4' shelter and a 4' X 5' run. Does Anybody else have a chicken tractor? I would love to see other designs.
I also used a system that was a 2x6 in my case with the wheels attached to it slightly longer than the width of the tractor to allow the wheels to turn freely.
Then I merely attached hinges to the board that enable me to flip the wheels under the end of the tractor when it needed to be moved; Then flipped out when the tractor was at rest to caress the earth. Placed at the heavy end of your tractor I would suggest.