That's what I was thinking.
Possible these are aftermarket gears that came apart?
He bought the jeep with the factory 3.55's, then found this set of 4.10s which we swapped in a little while back. So obviously we know nothing of their history. I'm also confident he didn't inspect the gears before he threw the axles into jeep. I'm confident I mentioned it to him, but I'm doubly more so confident that he didn't do so.
He's my best friend, but XJ owners gonna XJ.