My '70 F350 dually rides surprisingly smooth, with or without pulling 9k lbs behind it (or the 700lbs of fuel I had in the bed this time..lol). It rides MUCH smoother than
any of the newer F250 or 350's we have at work, loaded or unloaded, including a 3500 Chevy and Dodge that we have, all single wheel axles. By comparison, my '68 Highboy rode extremely rough until you got about 2000 lbs in the bed. Of all the full size trucks I've owned and driven, about 7 over the years (NONE newer than my current '70), the '70 dually is the smoothest...go figure.
The bad points of my F350 are the short wheelbase, which has a tendency to do the bouncy see-saw on certain roads, at certain speeds. Usually speeding up or down by 5 mph makes it go away though. And potholes. Man, those suckers send a jolt through the whole truck!
The old Econoline's suck in the ride department also. Sitting above or in front/behind of an axle is NOT a good combo. Not to mention feeling like you're going to slam into something face first if something pulls out in front of you