Personally I dont have back issues, my father on the other hand does. I cant really speak from my experience but I can from his.
He had 2 slipped discs which he had surgery for and on and off back pains in general, prior and post surgery. We had a 4th gen runner and we are now getting a 5th gen. He went for the limited trim for both models. I had asked him why not the SR5, my brother had a 3rd gen SR5 (either 3rd or 2nd i cant remember), my dad told me "NO! Drive then SR5 then go drive the Limited, the SR5 is a much rougher ride and is murder on my back but the Limited is MUCH better"
Im not sure if the seats are much different from the SR5 to the Limited and im speaking mainly about comfort while driving, which i guess can be linked to the comfort of the seat as well?
I plan on test driving an SR5, or maybe my buddys SR5, he was in the middle of purchasing one, maybe then I can have better input.