getting a shaking when im at a certian throttle postion
 

when im driveing my truck down the road it wont do the shaking but as soon as i let of the throttle i will feel it it only happens when my truck isnt under a load like if i and going down a bridge and let off the throttle not completly but just enough to maintain speed it will do the shaking then as soon as i give it gas it will stop could it be my ball joints i have no clue please help thanks.

jack the truck up and have somebody try to move your front tires up and down with a pry bar, just watch everything on the inside while they do it, with your hand on the upper and lower ball joints you'll feel any movement, and you'll be able to see it too. Also check wheel bearings by grabbing top of tire and pushing in and out, any play there could be wheel bearing. Just how is the condition of the bushings up front? Especially track bar

How are the U-Joints?

i have been told already that i need to change my ball joints but why would it only happen for instence if im going down the highway going 50 and i keep my foot on the throttle but let off just a hair it will do it then i give it gas and it quits every time i ease off the throttle no matter what speed it does it i can feel it on the floor and wheel