If you have an IWE issue(4x4 hub) and you can't put it in 4x4 at that speed just unhook both vac lines at the hubs, this will engage them and just drive the truck to see if the noise goes away. If the noise is gone one of your hubs is grinding. There are a lot of things that can cause this so don't just throw hubs on it until it's diagnosed. I'm a tech at a Ford dealer and I deal with these all the time. I've seen lots of bad solinoids, check valves, bad vacuum lines and IWE's so any of these can cause hubs to grind. PM me if you need any help with this, I know the system like the back of my hand.
