Ride-rite air bags. Easy no-drill install, you can inflate/deflate to just the right level angle you want (allowing varying loads to balance), easy to take care of. I had a 2012 F350 and 2011 F250 and put Rite-rites on both of them. Not sure why you get so much sag with those Fords but it is a problem. You especially notice it when you tow at night and everyone flashes their lights at you, because yours are pointed up into their eyes, LOL.
FYI all of the issues I had were within load specifications on the axle, load, etc. as well.