This is exactly my issues. Mine starts at 74...and then at 77 or 78 it is really noticeable. But the thing that baffles myself and my buddy who is the sales manager is that its not constant, I could go over the same road at the same speed and it would be different almost all the time. They took my wheels/tires off of my truck, put them on a brand new truck...no vibration/shake. Put the wheels/tires from the new truck on my truck...vibration/shake. Put the wheels from the new truck back on the new truck...no vibration/shake. My truck now has 30,000 miles on it, when it was new, it didn't have the vibration, it started when the truck had about 14,000 miles on it. At that time they said it was the tires and I was out of warranty of the tires so there was nothing that could be done. But I have kept pressing the issue and they started looking at it more closely. At 60 mph, its very smooth and drives great, but anything over 70 and it just sucks riding in it. Between this and the mylink issues, the crappy shifting transmission, I have not been real overall impressed with this truck. While the truck is being worked on, they have me a 2012 Silverado LTZ crew cab, with 15,000 miles. That things shifts so much better then mine its quite sad. The throttle response is so much better then mine, I think the software in these new trucks are design with maximum mpg and it compromises the driveabilty of the trucks. Ride wise, its no where close to as nice as mine, besides the shaking.