To check the sensor, pull the pump and check resistance across the two wires as you move the float arm all the way from one end of travel to the other. I am not sure which end of the travel has high resistance and which has low, but you will know right away. You are lookihg for a steady change from high to low, or from low to high as you cycle the arm. If it gets to the 5/8 point and the resistance does not change from there up, the sender is defective.
The only other thing I can think of is the pump is slightly out of position, and the float is hitting something. Try seeing if there is any play in the location and see if you can't change the position relative to the tank. Try a couple of positions if there is a large range of motion. That will be easier than pulling the pump.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)