Sagging rear - Replacing spring springs with stiffer ones?

[smassey321]

My 09 35d w/ Adaptive Drive sags just a little when I tow my boat. I recently switched to non run flat tires and things are a softer in general even with the 19" tires in the +40 psi range. I am wondering if I can switch to a stiffer rear spring to stiffen things up a bit?

Ideally I would like to add the factory self leveling rear suspension however nobody has successfully done that yet. I would even settle for a manual system where I just added air when towing (keeping the non self levering springs). At some point I need to crawl under there and see it there is any wiring at all for the factory system. My E70 is fully loaded and only missing the 3rd row so there is a very slight chance enough of the wiring is there.

Any advice is appreciated. Thanks