Im not familiar with the dodge truck fuel system but in most cases it is the fuel level sending unit/rheostat in the tank itself. Normally the sending unit is part of the in tank fuel pump but, like I said I dont know that fuel system to well.
check all around the tank area that external wires not mouldy or broken yes you may have a faulty float unit , best bet is to remove the tank and get a auto electrician to check it with a meter it takes little time other than that you will probably need a dash removal
