We wanted to love our first gen Sequoia, but the vehicle just sucked by Toyota standards. We thought an 80 series would be too impractical, but the difference between them is night and day.
I'm very familiar with the IFS woes of the Sequoia. A terrible design choice, indeed.
You can certainly go down this road swapping a solid axle, but I would highly advise getting a 80 or 100 series instead.
The Sequoia handles terribly, even in stock form. The hatches fail. The engine and transmission are great, of course. But the 80 has enough power in stock form. You can upgrade an 80 and it's well within its abilities. Upgrading a Sequoia is asking for trouble.
Maybe a 2nd gen Sequoia would be better for you? I don't know how they compare. After having many Toyotas, the Sequoia was by far the lowest quality rig we've had. It'd be good for in town use and light trails, but not any kind of regular abuse. And even in town, the handling is terrible!
If you need something just as big and better built, I'd see if there's better domestic options. Or maybe you'd like a 4th gen 4Runner with a V8?