I have 2015 8 Speed Sierra Denali and it has same problem, when it shifts down sometimes there is a hug hit from back like other car hits you from back, I sent the truck to the dealer and they recommended to change the transmission heat fluid sensor, then the harness but without any benifit, After that they recommended to open the transmission and change the transmission discks.
I have changed all discks as they recommended but no thinge changed, Then they checked the programming and the throttles.
Finally, they admits after all this money lost that it is a common issue is some trucks and they call it clutch bumb from back and there is no thing to do.
