That should do it.
Parked the Chevy nose-up on a hill, only to find a stream of ATF out the back, coming from the vent hole on the driveshaft yolk.
Apparently Chevy wants the driveshaft yolk splines greased, but the ATF would wash all that grease out. So they have a cup sort of thing on the tail shaft, with an o-ring to seal around the yolk. There is still an output shaft seal on the tail housing to keep ATF in. Because the yolk will move in suspension travel, and we cannot pressurize this cup thingie, a vent hole is located in the yolk end to release any pressure. This works, until the o-ring leaks.
While I had already replaced the o-ring, clearly something has gone wrong. Instead of removing the tailshaft extension again, I disassembled the driveshaft, brazed the hole closed, and reinstalled it.
Yay! No more leaks!
Should I have done that? Sure! LOTS of online suggestions, as well as even ~specific~parts~ you can buy that plug the hole.
Read More: The Crusty Chevy