My 5.4L will just be driving fine and then when I get off the highway after driving over a hour at about the second stop light will shake and knock really hard for a bout 3 seconds and then go away, it also does it at other random times at speeds less than 30mph, I notice it the most when I have been on the highway for awhile and get off. It had cam phaser, lifters, Cam’s, timing kit all done about 30,000 miles ago and truck has 201,000 miles on it.

 

Could be anything it could be the knock sensor going out it could be a issue with the fuel system not getting fuel or anything

 

It would be a good move to have your vehicle checked for any codes in it's memory, and go from there.

 

I would also have it checked out. I also had a 5.4l 2009 fx4 that did the same thing. Had it checked out and got some sensor codes together with an issue with the 4x4 transition box. Didn’t get it fixed right away and that caused me an accident where the truck flipped several times and was totaled. I got lucky with no serious injuries but you never know. So please get it check out. Good luck and god bless