4nrMike,
these old units have a switch array on the steering unit that tells the drive control what position the steerwheel is in. If one or any of these switches or the associated circuit wiring fails it can cause this very problem. These switches control the fwd/rev contactors telling the drive motors how to react when turning the steering wheel.
Normally when you turn both motors cannot drive at the same speed, the motor on the inside of the turn radius has to slow down to allow the truck to turn smoothly and not push out of the turn. In todays machines this is controlled by a wheel position sensor or encoder and the truck manager logic controlled the motor controller output, but back in the older days they were controlled by physical switches or LED pickups mounted on a plate or circuit board over the top of the steering assy.
Another problem may be in the fwd/rev contactors themselves so i would check both areas.