my 06 f350 dually, speedometer works intermittenly, odo doesn't work either replaced vss, didn't help, checked for power to vss, nothing

Does the odometer fail to display? This sounds like a concern with the instrument cluster itself, the power supply to it, the ground, the CAN networking, or related. Too hard to figure out without testing with a diagnostic tool that can actively control the components or view their operation status.