I have an extended cab 4wd 2000 silverado. The truck shake a little at exactly 20mph. I notice it more when i am giving it gas. I can feel the vibration in the pedal. Its fine once it hits 25 but will do it again at 60mph. I also make a weird clanking sound, usually when the truck is cold, which only happens at low speeds but goes away once i get moving. It sounds like it is coming from behind be. I was thinking driveshaft. What do you guys think. How can i troubleshoot it?