Rear end bouncing/Equal-i-zer 4-point 10K hitch

Throw the old rear shocks away; they are junk from the factory. Yes, I know it's a new truck, but the shocks are still junk from the factory.

Put a new set of Bilsteins shocks on the rear will help a bunch with rebound and that is the problem your having. (At least a lot of it)

You can make all the WD adjustments you want till the cows come home and it won't make any difference with control of the rebound of the spring control.

For some odd reason GM just can't get the rebound valving correct.