Wow.... Just wow....
Ok, I'm going to answer the OPs question since I've been down this road... some people actually buy a truck to use it for hauling....
4x4 Ranger springs will not lift your truck in the rear or increase the payload.
Air bags will work but are not cheap (you'll need the bags, brackets, and load controller with compressor).
I have thought about rigging up a set of overload leafs but it would require some fab work and honestly, after building my own leaf pack, I don't see that much of a need.
If you don't have a rear axle that has 10" drums, get one. You'll want drum brakes for hauling no matter what anyone says. Drum brakes don't stop you as quick as disks, but they handle heat WAY better (ever warp a rotor? how bout a drum?). Some 4x4s have a slightly larger master cylinder bore if you're thinking of upping your stopping capability. Unless I can find an even better solution than that, I'm doing that conversion and running DOT-4 brake fluid soon. It's overkill, but hey, why not error on the side of overkill.
Leaf packs.... Explorers are rated slightly higher than Rangers, IIRC, but not that much more because they're still designed not to ride harsh. They will lift the rear of a Ranger/Mazda pickup, however.
I wore out my Ranger leaf packs within a year or so of having the truck. After a couple years they were worn out so bad that I needed new tires and an alignment, but the springs in the rear had sagged bad enough that I couldn't get an alignment without fixing 'em. Around that time I managed to pick up a trac-locked 8.8" rear complete with shocks and leafs out of a 4x4 off a guy who was going to a 9" in his Ranger for $70. I decided that installing stock leaf springs were not the way to go, so I tore the leaf packs apart. I also got a set of leafs off a dodge that had bad spring eyes for free. Broke both sets of packs apart plus my old leaf springs. Wacked the eyes off the dodge main spring and placed it under the good Ranger main. Then added the longest Ranger leaf I had, then IIRC another shortened Dodge leaf and then another Ranger leaf, my intent being to keep the leafs as long as possible. I think I'm going to build a new pack for mine, however, and this time include putting a pack clamp on things to keep everything in line.
Other than that, it all worked great, ride was not compromised, it still rode nice and smooth, and actually I think if anything it increased handling slightly (I have no rear swaybar). I've had a LOT of weight in the bed with no problems either. I did have to use the shocks from a 4x4 Ranger, however, because the stock 2wd shocks only had to be compressed a half inch to bolt up, because 4x4s use 2" blocks between the spring and the axle, the shocks are a bit longer.
Over a ton of 2B limestone... stock springs.
After the custom leaf packs... not sure of exact weight but theres two motors (2.9L and 4.0L), A4LD auto trans, BW1354e transfer case, complete D-35 TTB front, complete 7.5" rear, spare axle shafts, two toolboxes (~100# total), Y-pipe, pushbar, radiator, misc small stuff, and an 80# bag of Portland Cement.
As you can see, it's sitting pretty level instead of laying way down in the rear.
