IRS has many virtues, just asks any Hummer H1 driver. The main reason you see nothing but solid axles in trucks is cost, the almighty dollar rules the roost. A solid axle much cheaper to design, manufacture and install. All the old school tow guys will scoff IRS because they think they are all made like a sports car rear end. But when done right are superior to a leave spring/solid axle, mainly the IFS tire/wheel location cannot shift around like a leave spring shackles and spring wrap will allow. A solid axle with 4/5 link/coil is a big improvement over the leave type as far as axle location rigidity and is the best for towing in my opinion. As far as off-roading the IRS has got better ground clearance as well as much less un-sprung weight for the tire to remain in contact with the ground. When I look closely at the Armada rear suspension I see rear wheel steering coming soon, which would be much easier to do to an IRS.
Throw in rear wheel steering and a locking diff, Nissan will be selling me another Armada. My uncle drove me around in his Denali w/ quadrasteer, man is it impressive. The ride was pretty comfortable, too. It'd be great to manuever in parking lots like my old Laser in something this big.