My ESOF hubs were not working, and the vacuum system was ruled out as the culprit. Further investigation reveals seal failures in both locking hubs, probably due to 11 years of fun. There are no service kits for these, and the part recyclers say they aren't available. Ford wants $296 each for new replacements, so the answer became replacement with manual locking hubs. courtesy of NAPA, $194 for the set. The install took 20 minutes.
The Ford ESOF system works on a vacuum signal transmitted through the knuckle and axle.
The Warn unit installs as 4 or 5 parts, depending on the truck you are working on. First there is the lock,
then a locking ring (which requires you put a 5-10 degree bend in the outboard tab)
then a axle retaining ring,
o-ring and the outer cover.
You start by installing the lock over the axle, then installing the locking ring with the bent tab to the outside, to facilitate future removal.
You need to check for clearance by installing two of the cover screws at 180 degree offset. Grab the two screws and pull them to see if the lock moves back and forth between the lock ring and housing. If there is no play, there are mods to do to gain the .12" clearance they require.
Note the amount of axle visible.
At this point, you need to install the retaining ring on the axle, if there is a groove on the axle to do so. Some Fords do not require this ring, mine was one of them.
Make sure the o-ring is installed on the cover and the cover is in the "Free" position. Install the cover and run the 6 screws in and torque to 44in#.
Repeat for the other side of the truck.
Now we have to disable the vacuum system. Find the Vacuum control solenoid, located on the passenger side firewall just behind the battery. There is a vacuum source line (red on my truck) and the line leading to the two knuckles (on mine, black).
Break and cap that line off, then test drive!
That's a temp fix until I get to NAPA for a new supply of caps. Now... queue someone with rebuild part numbers for the stock locks...