I figured I'd take a moment to update this thread in case anyone does research on these kits in the future.
I packed my tools in the truck last weekend, and noted that I only had 1" of suspension travel. While driving, I was bottoming out on the bumpstops, it rode like crap, and I know my rear wheel bearings probably didn't appreciate the jolts. I have to do the same trip in a couple weeks with the additional weight of a transmission, so I decided to finally make a decision and get it done.
I went with the Firestone kit for a few different reasons. First of all, the kit mounts closer to the axle than the Air-Lift kit, and it has a higher weight capacity. I don't intend on ever exceeding the Air-Lift's capacity, but I'll be happy knowing that I'm running well within range rather than maxing out. Firestone airbags are common in various aspects of the automotive world, so replacement parts should be cheaper and easier to come by.
I paid $280 for the kit, and first impressions were good. The brackets are sturdy and well-made and the hardware is decent.
Installation took more time that originally expected. The instructions are in plain English, but difficult to follow at times. It states to lift the truck and support the axle by jackstands. This is NOT simply a safety disclaimer. The airbags will not bolt in with the suspension at full-droop. If you want to install the kit with the help of a lift, you should chain the axle to the frame before you lift the vehicle in the air. (Also note that this will be required if you want to lift the vehicle after the airbags are installed. The bags can stretch too far and be damaged with the wheels off the ground)
It took me 4 hours to install. Part of that is because of the instructions, another part is because I'm a perfectionist. The most time-consuming steps were cutting off the rubber bumpstops, and drilling the holes for the frame. You will need a small bit for the pilot hole, and a 3/8" bit. These MUST be high-quality bits, or you will dull them on the frame. If you are using a cordless drill, charge up all your spare batteries because you will need them.
The bracket for the frame has four holes, but only two will be used. I recommend a diagonal pattern to avoid weakening the frame anymore than you have to. My truck has the load-sensitive brake bias, and the bracket is located right where you need to drill the holes. The nuts they provide have long tabs, and you have plenty of room inside the bracket to fish the nuts in and tighten them down. Don't worry about clearance, as you will have plenty of room. Note that you may want to adjust the rod for the brake bias so your rear brakes do a bit more work with the rear loaded.
I hit all of the brackets and the frame with undercoat. I suggest you do the same.
With the Leer cap installed but an empty bed, I found that 14psi was more than enough to bring the truck up to correct ride height. Pumping the springs up more (20-30) will provide an additional substantial lift. You wouldn't know it in this picture, but my truck has an engine sitting in the bed:
The truck rides much smoother, with less of a punch to the kidneys when hitting large bumps. I did note that the Rancho shocks do not offer enough dampening for the stiffer suspension and heavier loads. I was considering upgrading to Bilstein suspension all around, but I need to do further research to figure out what will be a bit heavier duty than what I'm running now.
I hope anyone reading this thread will find that useful. I'll try to get some pictures of the installed kit when I have the wheels off at the 300k service.