If I were you I would take the existing springs (if they are good, not broken) and remove some leafs. There is some guesswork involved on how many to remove. I actually replaced my springs and went with lighter ones, not a cheap option. I think I spent around $700 and a considerable amount of labor! I added helper air springs (5000 lbs) for when I'm hauling but I could probably still loose a leaf or two so the air springs haul more of the weight. My main reason for new springs was to fix a leaning bus, figured bad spring pack. I was wrong, looks like a bent frame. Expensive way to figure out...and the wife is still reminding me of that
The side I thought was 'good' actually had a broken main leaf. Air springs can soften the ride if your leaf springs are soft enough and the air springs are carrying some of the weight. I think my new setup gives me a little better ride, but it's always been pretty good with a 5000 vehicle in back.
More info on the spring replacement and photos:
http://trx.punknet.org/gallery2/main.ph ... emId=10496
Helper Air Springs install:
http://trx.punknet.org/gallery2/main.ph ... emId=10529