re: Silverado vibrating at highway speeds...sometimes.

If it's only doing it sometimes, either a bump could cause it via bad joints, or a bump plus unbalanced tires and sloppy suspension/steering joints could cause it. Jack the truck up and wiggle stuff around. If it moves, it's bad and could be a cause.

Joints are cheap at rockauto. Get either Moog problem solver or AC Delco professional joints. Do not put autozone joints on it. I have had many problems with them