Good news and bad news I guess. Good news is, it's pretty easy to get to that passenger rear quarter panel area. A few bolts and fasteners, just pops right off. There really is not much in the way of soundproofing back there. But, there's a lot mechanical/electrical stuff back there! Unbolting the Bose unit gets you into that quarter panel area. I didn't have any proper soundproofing materials, so I closed it back up.
Bad news is, I went down and drove a low mileage 2018 Armada at a local dealership. It had that SAME low resonance, booming sound over bumps at low speed. Maybe Nissan's right, maybe it is a characteristic of these vehicles. Knowing that doesn't make the noise/sensation any less annoying, but a little more tolerable somehow. Seems like I'm not the only one noticing it AND now I have first-hand proof that other Armadas share this phenomena.
They had a super low mileage 2015 Sequoia on the lot, I drove it too. I know it's iconic, it was overall a nice vehicle but much less composed, bumpy and interior was noticeably louder. I thought to myself, "if I am struggling with what I'm hearing in the Armada, I would really struggle with this Toyota." Honestly, at a similar price point, the 2nd gen Armada seemed MUCH nicer.
I'm not sure if I'm ready to absolve Nissan of what is obviously a design flaw.....so if there's any update out there regarding this issue, I would be glad to hear it.
In the meantime, maybe I'll look into a beefy set of tires on some stock 18in wheels to put a little bit more rubber between me and the road. Maybe that will change things a bit.