So, what are your engine and tranny temps? Expecting the tranny to stay at 170* while towing is unreasonable. I'm not sure that my Denali runs those temps empty on the interstate. With this tranny, I would say that anything under 230* (or possibly even higher) is safe.
I would also say that those claiming that the 6.2 is a "hot rod" engine and not meant to tow are just wrong. Most of the current pick up truck reviews state that engine in a Sierra Denali will out tow anything including the heralded EcoBoost.
In regard to the knocking, what octane are you using? The owners manual in my Denali recommends premium unleaded, and when not used, states that engine knocking is possible with lower octane fuels.
