My 99 F150 got stuck in 4 HI the other day (even though it was switched to 2WD) , and when I backed it up.. there was "clunk" sound and it came out of the 4HI. I took it to the local Ford garage, and they told me it looked like both solenoids had burned out, but couldnt tell why yet until they got new ones put on. Anyone else have this problem? Does it have anything to do with the T-case motor and GEM unit?

 

Odds are the t-case was disengaging, but the vacuum solenoids that engage and disengage the front axle Center Axle Disconnect could certainly cause the problem.

-Joe