In fords defense, they never needed to change it until recently. The first irs design was a double wishbone unit that allowed better handling but was too difficult to fine tune. Ford changed it to a multilink setup to give the engineers better flexibility, specifically for added weight and size of the EL. The current design is old, but at the time was unrivaled for complete dynamics(good handling, great ride, cargo flexibility and capacity), atleast in expys class. The nav gained competitors from Europe and their suspensions were very over engineered in sacrifice of durability. When ford added ccd over air suspension, it allowed even same levels of comfort without the price of air suspension. All ford had to do was tune the current setup for the 4th gen, but I believe the added size coupled with lower weight made it too difficult to get the dynamics just right with the budget of the program. As a result, though comfortable, there's problems with it that is clearly vocalized. Though the fourth gen I owned in general was better than my third gen nav, I dnt remember my nav having the top heavy rocker motion thats noticed on here. It also had a very controlled bounce as well. In retrospect, the body control on the air springs was extremely good for such a big heavy truck. I think the extra Weight actually helped the design as it kept the body planted and resisted excess body motions. On std springs, the 4th gen had good body motions as well but the rocking motion was more pronounced, I remember testing a ccd platty that had a rocking motion but it was only in low speeds. I don't know how ford is going to address the issue. I honestly don't believe they are changing the design much as ford has a history of doing the bare minimum for the expy. It took them a decade to give the expy the remodel it deserved, and though the product was fantastic, still had problems. Still, the irs design, though aged, is still world class. The basic design is 15+ years old, yet the ride and capability it gives is still competitive with products that have newer designs with minimal adjustments made. It's telling how good it is because it took gm deconstructing it to clear the bar ford set, and even then, in most instances (unless you get both the magnet ride and air springs) has not cleared it in terms of comfort and capability.
