Scooter,
The springs are usually attached to the differentials with u-bolts. First thing you want to do before anything is measure fender lip to the floor at the wheel center on all four corners, this way you have a reference point. You can support the truck by the frame and put a jack under the rearend to help hold it up. Once you have the u bolts off, you sill see a center bolt holding them all together. I usually put a c clamp on them for safety before I take out the bolt. You can take the bolt out to remove springs from the spring pack. You might have to get a shorter bolt to put the pack back together. This will make the truck sit a little lower but you will use air springs to raise it back up. You are correct, I would use Firestone air springs. You could do a weld on setup or buy the Firestone Ride-Rite kit for your truck.