This is either the IWE ( Integrated Wheel Ends ) solenoid or the vacuum lines / check valve / reservoir for the vacuum system
This is a block diagram of the IWE vacuum system
There is a TSB on the IWE solenoid for the 04 to early build 06 MY F-150s ( right click save as ) http://my.voyager.net/~sscully/file_tsb06_08_15.pdf
This is for the early build where water can get into the IWE vacuum system, causing weak vacuum at the IWE, and the spring is part engaging.
With the engine running, pull the IWE vacuum line ( not the vent line ) and measure vacuum, to make sure it is being applied. This is done with the Mode selector switch in 2WD and the engine must be running ( in park, take safety precautions, chock wheels, parking brake, etc )
Until you can get it fixed, pull the vacuum lines on the IWE ( make sure it is the vacuum line, not the vent line that runs into the engine compartment, and just hangs up there ).
This is so the IWE gear is not being ground down while driving it. The front axle is engaged when you do this, so MPG will suffer, but the truck is just pushing a dead front axle, the transfer case is not turning the front shaft. This is just to try to minimize further damage to the IWE gear ( which can cause a need to replace the IWEs ).