Pete's link points to a massive version of the rubber-sprung shackle idea which I mentioned earlier, known as the RL (rubber leaf) system.
The corresponding MOR/ryde product to the Dexter E-Z Flex is the
RE (rubber equalizer) system. It is much larger and more complex than the Dexter product, so although the Dexter system may be less effective, it should be much cheaper. By the way, the Dexter site features the E-Z Flex on the home page, but I don't see it anywhere else on the site, so it looks like getting more information means e-mail or a phone call.
As equalizers, these devices have little response to one axle moving up while the other one moves down, so a one-axle bump should have
less effect than both at the same time.
I still vote for shocks first: they won't interfere with any other plans, and may avoid the need for anything else. If it were me, I would add the shocks first (as I have on my Boler), then the trailer with the WD system disconnected - it may be smoother riding and well controlled without all that extra stuff.
By the way, MOR/ryde also sells something like the "rubber torsion" axles common in eggs (but using shear blocks) as the
IS (independent suspensions) system. They seem to be determined to find as many applications as possible for the rubber-in-shear blocks.