Since we now know that there are internal actuators in the 4.2 back boxes that open up with exhaust pressure, I wonder if these invariably seize up, since there's going to be no lubrication in there.
I got a rattle in my 4.2 back box after installing a x-pipe. Guess the flow was a bit too much for the old thing to handle. I did get it cut open, but not to see what the source of the rattle was. Although there is not really an actuator that can rust solid. It's just a spring valve that open/closed based on air pressure.
If I ever find the time and think about it I will have a look, still have it lying around somewhere.