Jesus,
How about this: a dual, full-hydro steering with a separate ram for each rear axle.
Bolted to each tie rod is an old-fashioned manual rack-and-pinion unit that is bolted directly to the axle housing. Any lightweight application would do, Ford, Chevy, etc. They can be had for about $100 new. There are even lightweight billet units out there for racing applications.
The 2 rack-&-pinions are connected to each other by a splined driveline (nothing big, just enough to keep the axles in synch) to allow for independent articulation of the axles. One of the pinions would have to be driven from the front side. So some modification would be necessary. Something like this would work. It's already set up to flip the pinion (at least that's what it looks like)
The hydro actually does the steering. The rack-&-pinion simply links the two axles together so they do not become out of synch angle-wise.
It sounds doable & relatively cheap. And it solves the Mudguppy paradox.
Different turning radii might pose a more creative solution.