well the only way your going to get your truck to carry more weight is put stiffer springs in or helper bags. if you get lift springs with the same spring rate, its still going to sack out in the back when you put gear in it. by the sounds of what you want, air bags are your best option. you can pump them up when you carry some weight. and when your empty your ride quality will be the same as it is now.

my springs/shocks are probably the stiffest anybody has ever seen in a pajero/montero, it is pretty rough on the road but when it comes to off-road it still rides like a couch. but either way it does not give me a "stiff neck".

I've had my pajero loaded with 5 people, overnight bags for everybody, and the roof rack loaded with skis/snowboards and it didn't squat too bad, I could definitely feel the weight as it hovered like a caddy down the highway but it still rode pretty good for how loaded down the B!tch was.