The set up sounds much the same as my Rover Rancher. It is hard to imagine how a worn pivot bolt would allow the wheels to "push forward" instead of turning especially in only one direction if it is similar to the Rover. On my Rover there was a crack in the frame just behind the pivoting member on one side that caused crazy steering. It developed because the bolt that moves up and down in the slot had come loose. A new bolt and Nylock nut and a splash with the arc welder fixed it.
