It also could be the parking brake. If you don't use the parking brake regularly it tends to rust and then when you finally do use it, it gets stuck partially engaged. This then causes the rear wheels to shake. You can check to see if this is the problem by driving down the road for a couple miles with the truck shaking. Then stop and feel the rear wheels and tires to see if they are warm and if you can smell the brakes.
You can also stop apply park break back up a few feet release break and it should free up.