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

How many miles on the tires? Are these OEM? If it happens only at highway speeds it may be balance which usually happens in a narrow band, for example 72 to 76 mph. I would have a trusted tire shop check for balance and while they are at it it is easy to check for slack in the steering and front suspension...Another thing you could do is rotate front to back. If the symptoms do not change I would think it is not in tires. If it then only vibrates in the floor/seats but not steering wheel, then it probably be in the tires.

Also if driving down the highway and it is vibrating , if you gently apply the brakes with your left foot while maintaining speed with your right foot, see if the vibration goes away. If it does it is probably slack in from suspension or wheel bearing.

Bad tire

lug nuts tight?