After reading this thread I can say that my 2014 Sierra does have less body shake. My 2012 Sierra had uncomfortable body shake driving over rough patches of roads. I realized, now, that my 2014 Sierra doesn't have body shake like the 2012. I am guessing the trade off is now that I get this resonance occurring, driving on smooth roads above 70mph. I live in New Jersey and don't often get a chance to drive above 70mph in daily driving. I agree that this resonance is intermittent and doesn't always happen. I've driven the same route some days and no vibration, other days the vibration in the truck gets a bit scary. I'm going to try changing the tire pressures and see what I get, the dealer delivered the truck with tire pressures of 35-38 psi.