I know this is a old thread but I purchased a 2014 GMC Sierra. The issues you described Are identical from what I’m experiencing.
It seems like even at a dead stop switching into reverse or forward there’s an initial clunk noise. As well as random shifting annoyances.
Of the few other trucks I’ve owned as well as other cars in my household I’ve never experienced this issue.
I kind of think it’s getting worse and wonder if there’s something that can be done like adjustments or possible worn parts in the rear end, trans etc.