I have a 2007 F150 5.4 Triton 4x4 automatic with overdrive. When I start off slowly and raise the throttle slightly the transmission then shifts through the gears all the way to overdrive and I am doing around 10 to 20 MPH. Then when I slowdown with either the brakes or just letting it coast to a stop it has a violent vibration. I have also noticed that when I am traveling around 50 or 55 at about 1400 to 1500 RPM there is a slight vibration in the truck. Flushed the transmission and changed the fluid 2 times within 12000 miles with no change. The truck has 137,000 miles on it. This does not happen all the time but has been getting progressively worse. Any help would be appreciated.

I forgot to mention that there is no check eng light nor any codes.