I think I would first get the steering end up off the ground and see if the binding goes away or is related to the position of the wheels and/or steering hand crank, then, as this is an electric [battery motive] forklift, I would be checking the power steering motor brushes as well as other failure points in the circuit to the power steering motor, like the PS/relay contacts and cables to the power steering motor.
If it has 2 hydraulic pump motors and one is smaller than the other, and the smaller one runs when the truck is in a driving mode, the smaller one will be the PS motor, and in that case, don't bother looking for a priority valve.
if the problem seems to be related to where the steering hand crank is but not in relation to where the steering wheels are pointed, you most likely have a defective/bad "orbitrol" (sometimes called the steering torque generator). If it seems related to where the steering wheels are pointed, you most likely have a mechanical failure in the steering axle.