Q: There is a vibration in the front driver's side of the truck at 38-45 mph

There is a very bad up and down vibration in the front drivers side of the truck between 38-45 mph. Below that speed there is vibration but not an extreme up and down vibration. Above that speed, it runs as smooth as a new truck. The vibration is a very hard up and down motion that causes the whole truck to shake. I have had the pitman arm, ball joints and wheel bearings replaced as well as the tires balanced and rotated. 2 different mechanics cannot figure it out. My steering is a little loose. I also discovered that while the vehicle is stationary, if I rotate the steering wheel all the way to the left and then shake it back and fourth, the truck acts as if it has a hydraulic system installed, which it doesn't. Doing this makes the truck bounce up and down about the equivalent as the above described vibration. Other tests I have done to try to locate the issue is accelerate and decelerate on straight aways and curves both in 2WD and 4WD HI. The vibration stays no matter how I drive.

My car has 210000 miles.
My car has an automatic transmission.