These are hdpe skid plates to protect the vulnerable control arm mounts on the frame of Jeep TJ Wranglers. For the factory or short arm mounts on the frame these bolt onto factory holes and provide a buffer for rocks to prevent the bending of the "wings" of the mount. They also are made of high density polyethylene, just like the big boys use on their rock racers. This stuff allows the Jeep to slide rather than stick to rocks. I have used my prototype set for 8 years of hard abuse before one finally gave out.
Jeep TJ "knee pads" skid plate