my truck starts to shake when i reach 20 mph. what might be the problem?

this actually happened to me not too long ago, could be a ball joint, my problem was my front bearing hub assembly, but that comes with some noise.if you have oversized tires they could be out of balance. could be as stupid as a loose lugnut. these are just suggestions, im no professional, but i've encountered my problems.

Ronny's on the right track, it could also be a worn U-joint.

thanks guys. ill let you guys know when i find out what exactly it is.

assuming you still havent found the problem, you might check the u-joints on the drive shaft, the same thing happened to me yesterday infact, except i didnt get much of a warning, truck started to vibrate real bad and my driveline hit the ground, and i was only doing 15-20 mph

What shakes ? The whole truck or the steering wheel ? Check the front end if steering wheel if whole truck check the tightness of the wheels or condtion of tires . If all ok then go for the U joint check