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It probably is the fuel level sensor. It could be sulphur deposits on the sending unit. Chevron Techron Concentrated fuel system cleaner (not the Pro Gard injector cleaner) is said to work wonders on sulphur deposits. Maybe you could try a couple of tanks of gas with Techron first before you R&R the bed and fuel pump. If it is the sending unit, you can replace it separately from the fuel pump. If it were me I would just do a whole new fuel pump anyway and be done with it, depending on the miles on the truck.