Update, I scored a cheap Pelican 1700 off ebay. I am going to make a drop down tailgate this winter and bolt the 1700 to the tailgate in an inverted position. That way I can drop the tailgate all the way down and then open the case 90* and have it create a "shelf" verses just dumping everything everywhere.
I picked up a set of windshield hinges and the spacing of the hinge from the mounting surface will allow me to clear the rear tire carrier/bumper and drop the gate straight down, eliminating the need for cables/chains to hold up the tailgate and allowing me to get closer to the tub.
We'll see if my theory works.
Here's what I scored... it fits JUST perfectly in the rear door opening!