On a ford fuel injected motor, like yours, stumbling at stand still and while moving can be caused by a few different things. First i would check the whether the mass air sensor is dirty. Not sure where it is located on your rig, but it should be between the air filter and the throttle body. Clean this and see if your problem goes away. It could also be caused by any number of pollution control equipment like the EGR or the throttle bypass sensor. Another place to look would be fuel pressure. Hope this helps.