The most likely cause is that either the left or right steer relay is stuck in the "closed" position. (Relay contacts are stuck in the closed position, even when the relay is not energized.) When either the left or the right steering switch is activated, the associted steering relay fires and allows current (through the switched circuit) to the "free-wheel" valve on the hydraulic manifold. The free-wheel valve stops the flow of oil from entering either the left or right drive motor, (in this case the drive motor that is not working) and the oil flow is diverted to the opposite drive motor. The problem can be solved by replacing the faulty steer relay. Both the left and right steer relays are found under the dash panel. (SPDT Relays)