For anyone confused about the front, there is a pivoting HDPE arm not mounted on there since we were missing a roller axle. This render shows the pivoting "ears" though which have a roller mounted on them. With a little surgical tubing to keep the arm planted to the ground and bolts as hard stops, the swivel arms pivot when the arm is touching the ground. Even in the render the roller isn't there, but you get the idea. Again, this is all just based off of wood prototypes.
http://i.imgur.com/ljvEQnG.jpg
One of the swivel arms:
http://i.imgur.com/3IPVcZY.jpg