2005 Gmc Sierra 3500 Fuel Gauge Problem

[bradpell8]

I have a 2005 Gmc sierra 3500 with a duramax. The fuel Gauge quit working on me last winter. I have since removed the body and sandblasted frame and painted. I replaced both sending units and lines. I know the sending units are working because when I unplug the front tanks sending unit the electric pump from the rear tank turns on. The gauge moves when the key turns on but goes back to empty and the low fuel light comes on. Any help would be appreciated.