I think we got it figured out -- am building brace and bracket that will mount under the bed and fit inside the hitch component under the rear end. It will connect just like a trailer - but sideways instead of backward ... and hook up with a round hitch pin. Will drill it out so the hitch pin can lock instead of a cotter pin ...
Will have photos, but am not sure how to post photo here -- will figure it out, or send photos to someone here and get it posted if anyone wants to see what we build -- but, after looking at the changes in the truck, it's not a big deal altho' it will take some fabrication, it's not difficult. It will be removable and installable without a torch.
the older model Happyjac will not fit - but it's only one front bracket we need to modify. But again, I don't like that bumper mount, so will develop a bracket and mount the rear - probably more like the Tork-lift mount.
AND FYI: The older version step-bars (Mine fit 2003 GMC) will fit the 2015 with a little modification too, the mounting brackets. Slipped right up there and bolted up ... but the bolt hole was out of alignment, so we elongated the bolt-slot and it snuggled right up tight and fit like it was made for it. Took ten minutes.