Is it this sound at about the 0:40 mark:
I have that exact sound, about to begin looking for vacuum leaks.
Based on the 4x4 system diagram (somewhere else on these forums EDIT: here it is http://www.f150forum.com/f4/4x4-trou...g-guide-39789/
) it sounds like the only thing you have left to replace is the vacuum line from the manifold to the selenoid, as you suggested.
If you think about it, that selenoid basically only opens and closes off the vacuum line. If you have a vacuum leak anywhere between the manifold and the IWE's, anywhere along that chain of hoses, it would effect vacuum pressure and probably the operation of the actuators.
EDIT: and looks like there are some check valves in the mix too, but I'm not certain what effect those would have on vacuum.