Dude, you can plenty of flex out of SUA. Did you ever fix your shackle angles? Longer flexy springs will help but just putting the axle on the bottom of your current setup will not make it flex more. Being tall and tippy will hurt you off-road, if you're doing it to fit 37s I understand but I'd take a lower armored, locked, geared wagon on 35s than a tall one on 37s.
Another thought, why not keep this thing as a clean daily/expo ride and buy a little 1st gen crawler that's someone has already injected cash into? You could buy a nice little reg cab crawler for $5k and it would wheel circles around a 60. You probably wheel a lot more than I do these days and people shouldn't tell you your business but if you're drifting to more hardcore stuff please take height into consideration, here's a cool thread over at that "other site":
http://www.pirate4x4.com/forum/toyota-truck-4runner/877646-low-riding-toyota-pics.html
And I really dig your ride by the way, you're doing a great job.