I replaced the rear springs a few years ago, but they're already starting to sag. What's the best way to deal with this? Where should I get the stuff? I'd like to give it 2-3 inches of lift.

Take it to a truck place that does springs and have them Re-arched. Its better than having to replace them. I did it they were fine for 8 years. But i drag race mine and it was killing my 60' times so i went to the hotchkis front/rears. Good luck too ya.