Thanks again for a great, informative thread! Before now I had no idea how the newer 4x4 systems worked. Now, thanks to this thread and my dilemma, I'm pretty well versed in them. Mine had a bit of a twist I thought I might add...I may not be 100% accurate in my diagnosis, but it sounds good to me and it works now.
I have an '05 F150 Screw that I bought about 2 months ago and it started the grinding intermittently about a week ago. It did it 3 times that I know of (if the radio was turned up I couldn't hear it). Followed this threads schematic on troubleshooting the system and could only find a weak check valve, which I replaced. While going back over checking for more leaks, problems, etc. I lost vacuum at the actuators while in 2WD so I started working backwards from the actuators and when I got back to the IWE solenoid, I wasn't getting any vacuum through it. While checking different lines and check valves to see if I had hooked up something wrong, I suddenly had vacuum again. I decided to go get a new IWE solenoid and replace it but wasn't totally sure I was going in the right direction. After replacing it, I have had no more problems. From what I can decipher, the combo of the 2 contributed to my problems. As long as the IWE solenoid was working all was well, but at times it would not work and the leaky check valve would allow the vacuum to drop and then the hubs would try and engage.
Anyway, thanks again and tomorrow I get to try out my newfound intelligence again...my wifes '03 4x4 Navigator is grinding when you accelerate. The way she has been describing it I thought it was a pinging noise upon acceleration. I drove it today and its definately a hub grinding and has been for some time now and it doesn't quit when you put it in AWD or 4x4 high so I hate to know what I'm going to find when I get in there.