Front runner - Your one of the lucky ones. My last silverado shook so bad at 75 mpg that it could nearly shake a bottle out of the center console. At times, I felt it was actually unsafe to drive it was that bad. Some trucks are obviously worse than others. When I first purchased the truck, it was smooth as butter going down the road but as mileage was added, the shake got worse. The dealership balanced the tires more than once, I took it to a tire shop and determined 1 tire was bad.. Smoking gun I suppose or so I thought. I had the tire replaced and all seemed better at first, but was obviously still not 100% smooth. With my truck I would observe smooth the truck riding smooth, then start into a violent shake, then smooth out, and repeat. Almost like a harmonic issue. I rotated the tires and still had the vibration. Always from the rear no matter what position the tires were in. I switched tires with another truck and the vibration was still there. For that truck I believe the rear gear set or pinion gear was the issue. I never got it resolved and like others here, traded. My excuse was I wanted a crew cab to justify the trade. My new truck is smooth, but not as butter all the time which is disappointing. My new truck is more than tolerable and is no where near what the last truck was. More annoying than anything really because I'm so sensitive to the issue now. My wife or passengers don't even feel it anymore. I actually believe this time it is the tires. I'll run them till I need new ones and will replace them and hope for the best.
Some trucks have a serious vibration and it's not an easy fix. I would think that GM will see/find a real solution soon since there's more than 2 years data of dealerships diagnosing it. I do believe that alot of these trucks had bad tires on them and that's all they needed to fix the issue but others have a pretty violent shake that isn't from the tires. That's the real problem here.