It is most likely the sender. This unit is attached to the fuel pump in the fuel tank. If you are going to attempt this drain the tank ( or wait till you have driven it empty ) you will need the GM fuel line quick connect tools $10 at any parts store. Undo the electrical connector to the fuel pump and all fuel lines ( after relieving pressure ), undo the bolts at the rear side of the tank straps ( it is recommended that you loosen the exhaust hanger and let the exhaust rest on the axle but it is not necessary ) then pull the tank out. The pump is held in by a retaining ring that will come out fairly easy. some pressure will be needed to set the new pump in as it is spring loaded and be careful not to damage the O ring. Just reverse removal procedure for install. Normally I would say troubleshoot the electrical system first but the sending unit is almost always the culprit.