I installed air helper springs to level the rear. With that set up I can adjust the ride height no mater how much I have on the rear. I tow a lot and the air assist springs work great for that.
On my previous Ranger I installed add-a leaf springs to level the rear with the shell. I worked well as long as I didn't add more weight or tow. I ended up also installing the air assist springs and that gave me the adjustability I needed.
The rubber air bags do need to be replaced periodically but their life is at least 5 years and they are not that expensive. I always keep a spare too, in case of a blowout. I have blown one out when hitting a very big pot hole with a heavy load while towing.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.