This doesn't make sense to me unless there is some kind of flaw in the mounting setup itself, as Boasun suggests. While there's plenty of vibration and torque down the shaft, there aren't typically the shearing forces that would break mounts, unless you are backing down at full RPMs with 50 knots on the side and the boat is rolling 35 degrees side to side.
But that should only cause lubrication issues...
The fact that you've busted stuff back there before does indeed suggest a misalignment, but one that would be obvious by wear in the stern tube or the cutlass bearing, as with a bent shaft. You sound like a candidate for a thrust bearing and a universal coupler, along with vibration-dampening soft mounts, which take alignment out of the equation for the most part, and, while expensive, are cheaper than new transmissions and engine hauls to redo sheared mounts.
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