Yea, the car has a matrix web beneath. Can't remember who it was from, definitely not KB or Stifflers. I think it was an Ebay deal way back in 2005. But it's held up quite well. It's square steel throughout and pretty thick.
I've been running IRS in two different forms since 2016. The first one, largely a stocker with just a few upgrades, was quite smooth and nice. But that's when nothing was hard mounted like the current one is. The rear subframe mounts of this Kenny Brown IRS just bolt straight to the frame, and I think I made the mistake of punching out the front rubber pinch mounts and pressing in Delrin instead. I think I'll punch those out and go with a more forgiving poly. But yea, the diff is actually solidly mounted to the subframe, though quite awesomely you can't hear it at all and there's no back seat to be soaking up any sound. And I do attribute it to the CenPeCo gear oil (look up their vid on Youtube, it's pretty amazing lube).
I think this is a pinion angle fault and/or the tires (for the most part). Having everything hard mounted just exacerbates the sensations that if removed shouldn't be an issue. So I'll have to measure the angles and put the tires on a machine to see if they're still even balanced.