I don't know too much about what kills automatics, cuz I've been a manual trans guy since my second car, purchased in 1970. I've never had an HO 360 /727 combo. So
As to the planetary hub; you said shaft but I assume you meant hub; I'm guessing it was an aluminum one, and a high mileage one. So maybe that has something to do with it. If you really meant shaft, I have no clue.....
Now if you have a very low-stall TC then you are gonna put low energy into the trans, so Ima thinking the 2.76s are not the cause of the near failure.
Unless you were peg legging it. You weren't doing that were you? lol. Or neutral drops, to get it to spin .... were you?
Because that kind of shock-loading could certainly hurt it. Peg-legging doubles the rear axle speed on the side that is spinning. If that tire is alternately slipping and grabbing, (I mean you'll have to imagine the circumstances for that to occur) well; slipping and grabbing is like a jack-hammer to the trans.
As the stall-speed goes up, more and more torque goes into the trans, and with the driveshaft sorta held stationary by the 2.76s, hyup the torque goes straight into the planetary. But as long as you don't shock it with neutral drops or J-hooks,or other bad behavior,lol. it shouldn't hurt it. So IDK what killed it.