Try attaching 2x4s to the legs and use them as skids.
You should also bevel the leading edge so it doesn't dig in to the ground.
Exactly. Screw some 2x4's to the legs might even suggest some diagonal bracing out of scraps. Get a three round fence posts. Put two of them under the 2x4's and roll it adding one post to the front and removing one from the back as you go. If you don't need the posts when you are done, save your receipt and take them back to the store.