Axle shaft broke 8 3/4

It does look like an internal metal flaw that has been present for a long time.
I worked in primary steel production in Gary Indiana, and eliminating gaseous inclusions in metal was a primary concern, because this is one result.
The metal probably had a porous surface at one tiny point, leading to the interior..
If the rear end sat for a long time before you used it, the internal rust in the axle finished it off. It happens. Have the other axle dynafluxed, xrayed, whatever you need to make sure it's not partly fractured and waiting to do the same.
Store axles in a warm dry place with grease on them.