I have vibration but its pretty intense at 70mph.. to the point of where stuff rattles in my cupholders.. i think my driveshaft isnt balanced or something odd... gonna take it to the dealer soon.
http://www.amazon.com/19119199-Upgrade-200-Watt-Subwoofer-Amplifier/dp/B009AYPB7O/ref=sr_1_70?ie=UTF8&qid=1438202337&sr=8-70&keywords=2014+Silverado+accessories
$859 + free shipping from chevrolet on amazon.