The sending unit grounds to the tank and the tank is normally grounded to the frame.
On a panel body trucks, the sending unit is grounded to the frame through the medal gas line that is part of the sending unit and that is grounded to the frame at many places with metal hold-down clips.
On cab-body trucks, some people run a ground wire from a sending unit attachment screw to a clean ground on the frame (you could at least try this quickly see if that is the problem).
