If I read your description correctly, you devised a stiffer lower mount to reduce the shock load. Interesting, I might have thought a soft mount would soak up the shock more while a firmer one would transfer that load to the diff more. In other words a bit of the opposite effect. However I see where allowing the tires to spin would allow the load to essentially pass through the driveline altogether. But it seems the moment before they spin, when the shock first hits the system, is when things would break. Obviously your experience showed otherwise, curious to hear more details on how this worked to prevent failures.
By the way, I assume you still have a open diff? So we're talking about spinning one tire. Or do you have a limited slip diff?