I am not sure where it is in your truck, but these units should have a hydraulic manifold in which there are a reversing valve and a differential valve (there may also be some speed control solenoid valves there also). I am betting you have the wires crossed on the negative sides of the solenoid coils for these valves.
you may be able to figure out where these valves are by following the hoses from the drive motors at the wheels back to where they go into a hydraulic valve manifold.