2nd Gen Non-Engine/Transmission bumpy ride

[KCanepari]

I have a 2000 dodge ram 2500 diesel 4x4 105000 miles that I bought in the spring. it's a regular cab with a custom aluminum dump body. It rides really rough, not so bad with a load but the rear end hops empty. it doesn't seem to have any mods except for load stops on the rear and I have no idea how many miles on the shocks. . I'm looking for suggetions for a smoother ride, but also to still handle the load. I have to slow down to below 60 on the highway over the bridges or else I almost get all four wheels off the ground. I have a '96 2500 diesel regular cab stock bed 256000 miles and it ride 4 times better.