Is the steer motor mounted tightly? If i'm guessing the model correctly there are three bolts that hold the steer motor on, and they can snap. Also, check that the steer motor gear ios properly attached.
After that, when you jack the unit up how 'sloppy' is the main bearing that the drive unit mounts into. this can wear badly until the unit will not turn. Check these bolts (three again) and make sure tha the drive unit is tight with the frame.