My truck starts to shake when i reach 20 mph. what could be the problem?

Could be a multitude of issues, but we'll start with the easy possibilities first. --------- Check all of your tires. You may have a broken belt in one (or both) front tires and that can lead to shaking. If you can, take the tires off and look at each tire straight on, as if you were looking through the tread. Roll the tire slowly away from you and watch to see if the tread is off-center, or if the tire has a bulge on one sidewall. If so, that would be your problem. Just get new tires.

Next, check all of your lug nuts to make sure they are all there and all tight.(Front and back). Also check to see if any of your holes where the studs come through the rims aren't 'oval', as this is a sign of loose lugnuts, will cause shaking, not to mention, is extremely unsafe. If any holes are oval, you will need to replace the rims, and also possibly the studs as the studs have had excessive uneven torque on them, leading them to fail and you could loose a tire while driving. Good luck!