hello i am john a currently employed ford dealership technician
i hope i can help resolve your issues
there are several things that can cause your problem
pulse vacuum solonoid
bad vacuum pump (if diesel)
bad hub lock assemblies
i would first check to see if you have vacuum at the solonoid. it is located in the engine compartment on the passenger side fender behind the battery. remove the lines with the engine running and see if you can feel vacuum on one of the lines. if you dont have vacuum and it is diesel you need to replace the vacuum pump. it is located close to the blower motor on the same side of the engine compartment. if you do have vacuum you need to check that all the vacuum lines are hooked up. look under at the wheel spindles and see if the lines are hooked up to the spindles. they are black rubber lines. if these are ok you need to vacuum test the lines. you will need a hand held vacuum pump for this. if the lines hold vacuum then you need to check if the solonoid is working. if you feel this is too much you may have to take to a dealer.
hope this helps, if you need any more info just let me know.
One other question while i have your attention, I have fluid leaking out of the wheel hubs and have been told the tube seals are bad, and expensive, if I fill the race with heavy grease can I avoid replacing the seals for any lenght of time??
Your help on the vacuum is just what I needed.
you could let it ride for a while, but dont use 4wd. as long as you are not using 4wd and the hubs are not locked nothing inside is spinning so you dont need to worry about lube. but if you use 4wd alot... you need to replace the seals so you dont burn up the front diff.
hope this helps