When you price the potentiometer you'll not want to just throw one on there just to see if that's the problem.

Sounds like a combination of two adjustments incorrect.

Not raising all the way might indicate that the hitch control potentiometer is not reaching a value to tell the controller to raise any further.

When you raise the hitch with the rocker switch and it goes to high pressure indicates the hitch position potentiometer is out of adjustment.

First adjust the hitch position pot so that at maximum height there is still some vertical play left in the hitch. The hitch lift arms should be about parallel with the back of the fuel tank. Loosen the arm that is clamped to the shaft and turn the shaft to adjust the potentiometer. This adjustment should not be done with the engine running as unexpected hitch movement could cause serious injury.

Next adjust the position control pot so that, when the handle is in full raise position the hitch is at full raise. 

The hitch should also lower completely, the last little bit of downward travel might be a little slow.

This will get you close.