I am actually considering going with a full manual hub, the vaccum systems on these have a lot of weak links, my whole system is new including the hubs and at times the hubs just won't engage, I think if you just have a little bit of moisture/condensation in the lines they will freeze up, it doesn't take much, If I know I am going to need 4wd I have started just locking the hubs manually before even leaving the driveway, it's the only sure way you know your going to have it.
2008 F250, 4x4, ext cab.