My 2014 LT Crew Cab 6.5' is in the shop for the 5th same issues as most of you.
It had a vibration at 42-50ish and then at 74ish also under acceleration at about 75 it makes a loud vibration noise.
Tires have been road force balanced 3 times, they even ordered new machine to make sure.
New Tires seemed to help for a few weeks then it became worse, thought i was just going crazy. (18" GoodYear)
Took it in yesterday, today they called stated that they scoped it and the rear end has a vibration they will be taking apart and checking the sepcs on the current rear end and replacing it.
I would like to note that my dealership has been nothing but helpful, this is obviously a well known issue by now that GM has to know about with all these field "engineers" testing vehicles. Its also obvious they hope as most people are oblivious to the world that they will never notice this issue to save them $$$$$.. I on the other hand expect my vehicles to be 100% and especially one with 4000 miles and it has been doing this since day one. I will be going down the lemon law path on the next visit I can't afford to be paying for a broken vehicle.