You talk about what you tow, but not where you plan to drive.
I have a 4wd, but have rarely had to lock the 4wd in even on my ranch, and I think mine are open diffs. These truck are so heavy, if you end up getting stuck, I am not sure a locker would get you out.
Depending on where you plan to drive, I would likely roll the dice and see when If I got stuck. If I did get stuck, I would do what Justin suggested, get a front reciever, and a winch. I think this would be a better investment.
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