Brad, I've been giving your set up some thought.
With it sitting low, you'll have such little travel left before the shocks bottom out. With your old cones, they would have been so hard after 30+ years, they would have had all the 'give' of a besser block. Now you have cones that allow the suspension to move, it's working but in doing so, it only would have 1 or so inches before it runs out of travel and yes, when it dos, it will do so hard.
If you want to keep it low, there's no solution here, other than to live with it. You could fit harder cones, but that's not the solution. It will ride awful and handle poorly.
As I think you found with your experiment by raising the back, you now have more movement before the back bottoms out and the issue (if I understand things) has been over come. That's proof if you like of what I have mention above.
I'm not sure a Standard Trumpet is warranted in your case, it wouldn't hurt at all, but it will be sitting up a fair way. We run them - or rather the other guys run them - as there's no commercially available Rear Hilo that will stand up to what we do. I have made my own Adjustable rear Trumpets, but that's a story for another day.
One other thing that springs to mind here that would fall in line with when your Moke was built. I have come across this 3 times now in the very late Mokes. The Rear Subframes in these were made wrong. I really don't know how they could be stuffed up, but they were. The Side Rails in them were exactly 1/4" too long. Doesn't sound like much, but the ramification are huge. I have never spent the time on any of them to see exactly where / how this issue was created, but it's extra length is between the trailing arm pivot and the runner cone base. It's not that the hole along is in the wrong place, but that whole rail is longer. This flows on to have 2 issues;-
Toe Out that is way beyond anything that can be corrected, and
A very soft rear spring rate / sitting low at the back
Looking at these subframes,(when removed), The outer ends of the front rail were bent back. I did go to some trouble to straighten one of these and it wouldn't fit back in - the rear mounts missed out lining up with the bolt holes by 1/4".
I'm also just wondering of this may also be an issue with yours ?