I think you are spot-on with the comment of poor dampening.   Upgrading shocks on my Dweller, as detailed in
 this post, was the single most important upgrade I have made.  It totally changed the highway road manners in a very positive way.
I would not consider the Rancho RS55118 a cheap shock, but I think a single RS55118 is likely not providing enough dampening for this application.   Like most Rancho products, this shock is specified for a broad range of applications., everything from a FJ40 Landcruiser to a 1-ton Chevy pickup.  It would seem that the valving would need to be a compromise for both the very light and the very heavy applications.
The Cruisemaster ATX appears to have an extremely stout remote-reservoir shock on each trailing arm.