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

Lets assume balance steering and suspension are OK. I had that happen to me not long after having a tire replaced. The shop had water in the air lines and I ended up with a small amount of water inside the tire. It only takes less than an ounce to effect balance. As the water moved inside the tire from stopping and going, temperature, etc it would change the balance. We found out by breaking the tire back down and examining the inside of the tire. Have you had any tire work lately?