Most of these trucks have an overload leaf - if they're flat and/or sitting on that overload leaf in the rear, time to get some help.
I just added the add-a-leaf from Jegs.. forget the vendor - about $30 for both sides - levels the 4runner and provides a much better ride. I'm not ready to lift yet, but I think it's one of the best dollar per benefit mods you can do.
you still have some arch but not much, that is about as flat as you would want your springs to go, i dont know if an AAL will give you any arch. dcg could help you with that one but it would probably be helpful and be good for your purpose. wow i forgot how cramped the tire makes it under there