To raise the rear coupler height, I took off the shell and removed the motor to free the truck. By turning a loose screw, I was able to raise the knuckle coupler. I did not completely tighten the screw in order that the coupler arm could still swing freely laterally.
The knuckle still sits a bit lower than other brands but now is usable. BTW, when I had the shell off, I made the series-motor wiring mod implemented by several others.