[sirwired]

Unless the diff cover is made of re-purposed soup cans they bent in a metal press, I'm gonna go with "Some idiot cross-threaded it and then latched it down with a breaker bar." or "Some idiot thought crush washers can be re-used indefinitely, and when it wouldn't stop drippng, latched it down with a breaker bar." Torque spec on the plug is only 35 ft-lb; that's not going to strip threads of that size unless it was going into sheet-metal, which it doesn't.

Looking at a video online, it appears to be the cast aluminum you'd expect it to be. If a tech is being a moron, it's certainly both straightforward to screw it up, and straightforward for any independent mechanic to fix with a heli-coil or tim-sert. (Those things are routinely used for stripped spark plugs; I think they are up to the challenge of a little 'ol drain plug.)