The truck's transmission shutters, so the whole truck vibrates. this is a huge safety issue. it causes the truck to slow down and overdrive does not kick in. so you have to slow down and force the truck to shift. this is a 5.4 triton motor. i did change the plugs and coil packs. this job is not cheap. if you look at the ford forums i am one of thousands. if you shut the over drive off problem is fixed but you can't drive over 55 without toasting the motor and using a huge amount of fuel. it appears this happens very frequently. this should be a recall issue. http://m.carcomplaints.com/ford/f-150/2004/transmission/no_overdrive.shtml