too stiff of springs for the load is the usual culprit of the stiff ride at the rear of a trailer....
a spring that is too stiff is too much like a solid mounted axle...and one that is too soft will bottom out when an extreme bump is encountered....
one that is just right will have nearly full movement, and will be able to use that movement to minimize the harsh ride of the load by losing some of the jarring from the road, in the movement of the springs...
its not the easy up and down bounce/movement of the trailer that causes the issue, but the sudden jarring movement of it when a bump is encountered with stiff springs....
install softer springs so that the axles can have a little more movement, independently of the frame... this will dampen the shock that is transfered from the axles, thru the springs to the trailer frame, and then add shock absorbers to control the axle movement under the extreme conditions that commonly occur on our "modern" highways at the speeds we travel...