The major structural areas at the rear of your cab are the B-Posts/Pillars. The ribbing below the window does give it some strength, but they are mostly there to prevent that annoying metal sound you get, example: wobble a piece of sheet metal or sheet steel by hand. Also, the ribs help deaden sound. Just so you know, I used to do body an frame repair for a living. There are plenty of opinions floating around concerning structure and strength, you will have to decide whether or not you'd be comfortable driving the truck with your family in it. I myself wouldn't have a problem cutting it out if I were in need. I would make sure to reinforce the edge though. A camper boot could be installed to connect the cab to the bed/box, whatever you call it. It would give you a nice size entryway for a camper body. It's just a thought, something to ponder. Best of luck with your project.