I have a 2006 f150 FX4 with with a 4x4 engaement problem, when engaging into 4x4 high one of the hubs makes a loud bang, i can feel a kick in the wheel then the system engages. here is what has been done already by both my regular mechanic and a dealership
Vacuum system was checked out everything is fine
Passenger side axle, IWE actuator and Hub have all been replaced
Driver side IWE actuator replaced
Still same problem I have already put 800 dollars into this problem and the truck has 62,000 miles on it all i want is my truck to be fixed. any ideas??

 

... Read the manual on how to place it into 4wd... And are you testing on pavement

 

I know how to put it in 4x4, i have had the truck for 3 yrs and 50,000 miles never had this problem til now, no not testing on dry pavement never engage going faster than 40 mph like it says in the manual

 

My 2004 won't engage unless it in neutral , doesn't say that anywhere in the manual.

 

FAIL......if your truck will not go into 4 Hi while traveling down the road ... you have a problem .... now you must come to a complete stop and place it in Neutral to engage 4L.

 

I can't offer any help on the 4x4 engagement problem, but try posting your problem here: https://www.f150forum.com/f4/

you will probably get more/better responses in that section.
Good luck!