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Hi I'm a ford senior master tech and I would like to help you out. The best way to diagnose this concern is with a scan tool that can view live data for both the abs module and Instrument cluster. The two wires from the rear axle speed sensor go to the abs module pins 6 and 14. the vehicle speed is read by the abs module and then the abs module transfers the speed signal to the instrument cluster. Commonly what happens is the abs module fails internally and fails to send the signal to the cluster. you will need a scan tool to diagnose this. you have to drive the truck and read the vehicle speed from the abs module. It will most likely read ok . Then go to the cluster pid and read the signal there. if it reads 0 mph it is because the abs module fails to send the signal to the cluster. This is a common concern I've seen quite a few times before on superdutys. Without a scan tool the only testing you can do is to ohm out the circuits from the vss to the abs module. If they prove ok I would suspect an abs module. Let me know if you need further assistance.