E&Es rear lower shock mounts are the ticket but don't completely help as it's not pre-loading the spring. The Wescott rear spacer (I think 30mm?) is what's doign most of the pre-load help. To
@lx200inAR 's point, it's asking a lot from the system and with the cantilevered weight which is transferring a substantial additional amount of weight from the front to rear axle.
New spring could potentially help.
Alternatively, if you can shorten the hitch carrier so it's closer to the vehicle will also help.
I like
@teejnut suggestion the best.