Am I justified in my frustration?

If you lived near me, I would buy what's left of the diff off of you for your $350 and pull it apart myself.
I love a challenge.
And believe me, it would come apart one way or another even if I had to cut a window in the housing and cut the axle apart at the gear set to pop the third member out.
I had to do that once on a dump truck that had an axle fail in the splines.
It literally welded itself together there........