I have a 2011 F150 FX4. I changed the fluid in my transfer case, front and rear diffs a while back. I used all the proper specs as far as fluid weight and friction modifiers depending on which one i was doing. Now i am having issues. The problem doesn't happen all the time but as with any problem, it presents itself anytime you need it to work right.

Sometimes, it works properly. Other times when I switch into 4H I get the wrench and it doesn't change or it will drop straight into 4L regardless of speed. I have tried to do the switch at anywhere from about 30MPH to 0MPH with the same results. In order to switch back into 2WD I have to go back and forth a couple times and hope it drops back in place.

At first I thought it may be the vacuum actuator. So I figured it was a cheap part so I changed it out to no avail. I can feel and hear the vacuum from the actuator all the way down to the hubs.

I just have no clue what the problem is and to be honest I am not that good with diagnosing problems with 4WD.