what you are feeling is a normal charactarisic of the superduty vehicles...either loaded or unloaded depending on what you prefer.... The vibration can be fixed, but only one of two ways usually. What you have to do to fix it is shimm the carrier bearing down to correct the driveline angle. Once its fixed, it will do it either loaded or unloaded. For example......If you shimm it so that it is perfect unloaded, it will do it loaded and visa versa......unfortuanetley driveline angles are such a precise science that the truck would basically have to have NO suspention on it to not do what you are feeling......Ford dealers are able to get kits to fix this and there is a TSB on it. The TSB basically explains what i just told you, but longer....and gives a part number for the shim kit.