I don't know about mustangs, but that year superduty will do the same thing with different sized tires, gears, a loose speed sensor plug on the rear diff or a faulty speed sensor. As mentioned, get it scanned. I think ford stuff all goes throught the same OBDII plug but may take a different or better scanner, some others (chevy) have a seperate drivetrain diagnostic plug.
edit: I spelled diff : doff