I have a 2005 f 150 king ranch 4x4 with 90k miles... When cruising at speeds more than 20 mph, if I drop to passing gear, the entire truck shakes and I hear a loud clunking sound from under the truck. The Ford dealership found no obvious reasons....any ideas??

 

Have them look a little closer............did they even drive the truck?

 

could be your torque converter. when was the last time you changed your trans fluids and filter? also im not sure about your truck having a carrier bearing on the driveshaft but check that and your u-joints.