Those 2008-2010 Super Duties were just beefier in the suspension department. Here's my former '09 back when we had the Cyclone.
It sagged far less than the 2011 F450 that replaced it.
I bought a 2013 F350 two years ago for my business. It sagged so much with a landscape trailer, I couldn't drive it at night without blinding everybody. I took it to a local heavy truck repair business:
Fleet PrideThey did some measurements of the rear suspension with and without loads and made me some custom leaves for the rear leaf spring pack. Not only does the truck ride better, it handles heavy loads with ease. I will definitely do that again if needed.
