I've been having intermittent problems with my 4WD on my '99 F150. It has the SOTF switch on the dash.

Checked the 4wd solenoid behind the battery and there is vacuum in and vacuum out.

After checking for vacuum I had my daughter goose it in the loose snow, and the front axle was spinning, so it engaged today. Earlier this week I backed into a muddy area and the back tires spun, flipped it into 4wd and never could get it to kick in. Two days before I used the 4wd when the roads were slick, and it worked fine, as I felt the back tires spin going up a hill, but forward momentum never slowed.

Could this be the 2wd/4wd vacuum motor causing the problems? It seems to work better when it is colder.

 

Drove the truck today, and seemed to go in and out of 4WD as needed, had to shift in and out several times going from snow covered road, to clear roads, pulled right up a snow covered bank at my BIL's. Got home, backed into the field by the house in 4WH and tromped on the gas pedal, front end slung snow everywhere.

I'm thinking the actuator was sticking some how, and flipping it back and forth, applying vacuum one direction, and then the next, freed it up.

 

i have a 2003 f 150 4 wd seems to have the same problem. sometimes it will work and then sometimes under a load the 4 wd will not work. actuator moves back and forth and while on stands wheels will not turn. I have heard about problems with a plastic fork inside front diff.