I, too, can vouch for the vibration issue with the 2014 Silverado. Like some of you I didnt test drive the truck at highway speeds. I noticed the vibration as soon as I got on the highway headed home but it wasnt bad enough to turn around. After a couple days of it I started looking online to see if this was normal and I found this thread. It's been a little over 2 years, 68k miles, 2nd set of tires, numerous trips to the dealership, and the problem is still there. Each time they give me the same excuses of "can't duplicate it" or "its fixed". At first they kept telling me "its a truck, it will do that" or "well you have the 20's on it so it will ride rough". Yeah, especially compared to the Acadia I just traded in for this truck. But it still shouldnt ride like a tank. I've ridden in 18-wheeler tractors and H1 Hummers that rode like Caddy's compared to this truck. I've talked to co-workers and friends who have the exact same truck and they all love theirs. It's been long enough. I cant stand driving this thing anymore.
I am bringing it to another dealership to try some of the things i just read about in this thread and see if they can resolve it.