I had the Firestone's put on my 84' GMC plumbing truck this year, about $500. installed, and i couldn't be happier. They have kits for all models.
When loading and unloading my pipe threading machine, very heavy, i let the air out of the bags because that allows the cart to then be the same height as the machine and it will slide right on. After the machine is off, i'll re-air up and my truck lifts up about 2 1/2-3" in the rear with 50 psi air. I had the air zirks for each side installed between the tail lights on my utility bed, out of the way yet very handy. i find my truck to look and handle better in all situations with the bags.
Joey