I would look at a set of Timbrons. They replace the bump stops and prevent the sag. They only come into play when you have a load, $330 and 30 minutes to change out. Made a big difference on 4x4 Tundra pulling a 8K TT. A friend put them on this F-350 for hauling produce trailer w/ same good results. No maintenance. Look on etrailer.com.
I just watched a YouTube video about these. Looks easy to install and would do the trick for a lot less than airbags.