The CV joints are fine and the boots were not torn. There was slight rust on the shaft where it sheared, just where the outer boot is fixed around the shaft. Maybe there was a flaw during manufacture that created a weak point?
The shaft is 1" diameter solid steel so it should not break.

Will I have to drain the diff oil before removing the old shaft or is the carrier sealed?

I am getting a s/h replacement assembly to fit this weekend, I have heard the bolts fixing to the carrier can be tricky to remove, any tips?