We are eventually going to a C in the future, but certainly can not afford a 4X4 version. I have also been thinking of a rear locker but I am definitely going selectable. I have the Ford factory electric locker in the rear and I installed an Eaton electric front locker. Both have been flawless. In fact, back in my snowmobiling days puling an enclosed trailer I would just kick in the rear locker to get moving again if I had to stop on a grade with slick surface. This is with the weight of the camper being mostly over the rear wheels. Had I not had the TC I would of had to use 4X4.
So, a C being much like a TC with substantial weight over the rear axle I think a locker would be a worthy addition.