I took mine in to the dealer for the shuddering between 20 and 50 mph. They did a transmission fluid flush 8 times. Yes 8 times. That's what the manufacturer is calling for. My vibrations are gone for now. Hopefully this was a permanent solution.
My 2015 Z71 LTZ still stock.
On 275/60-20 Destination A/T's and Eibach Pro Truck Sport shocks all around. The front are on the highest setting ~2 inches higher than stock.