I have the 93 dually with the heavy duty 10 leaf spring packs. I think 7500lbs?
It rode really really horrible, almost like the axle was bolted onto the frame seemingly, but when I replaced a broken leaf mount and shackle it seems to have improved a lot. I think the old shackles binding/getting stuck had a lot to do with it. I would use a jack on one side and make sure yours are moving the way they are supposed to.
It also rode a huge amount better when I had a 16ft car hauler towing a vw bug with most of the weight on the hitch, as the hitch was about 6-10" too tall. zero squat. It was still a truck, but I think especially the duallys are supposed to have a literal ton of extra weight back there so they ride properly. I've thought about bags and such but as I am only using it to tow I wont worry about it.
It will never ride as good as my 83 diesel mercedes benz with blown shocks: that thing is like an air-bagged water bed on a pontoon boat.
-Q