Sending unit is bad. but as said check the wiring first. sending units in chrysler vehicles are part of the whole pump unit that is inside the tank. The whole assembly must be replaced. The unit does not cost too much, but the labor can be intensive...Good Luck
You're missing a good ground between the frame and the plow. Take a jumper cable and attach between chassis and plow and see if that doesn't fix it. Then attach any kind of insulated wire between chassis and plow.