Since the existing Montero IFS already has the rigidly mounted differential, there's little to be gained as far as unsprung weight goes, which is the primary advantage of a DeDion suspension design. What I think you're suggesting is some sort of suspended subframe, which would increase the unsprung weight without any benefit to the suspension geometry or travel over a well-designed IFS.

If it's never been done by any auto manufacturer, there's probably a good reason - either the complexity or the expense of the design makes it unrealistic. Usually this means that better results can be obtained with a more commonly existing design. I'm not discouraging thinking outside the box, but when it comes to off-road suspensions, I don't think there are many possible designs that haven't already been tried. If you want the ultimate in an off-road suspension, look to the vehicles that are built to compete in whatever format you're thinking of using your vehicle for (rock-crawling, desert racing, rallying, etc). The winning vehicles in those arenas will have the best suspension designs for what they are trying to accomplish.