I'm happy to see serious consideration given to this topic; it's made me a little crazy for some time, and we may not get to the bottom of it for awhile yet. Thanks for sharing your experience and acuity.
Particulars in my bout with the disease are that several members and I disassembled, cleaned and inspected a very decent 3.46 axle after PUB 09. The diff assembly per se looked to be 100% and Joe and I left it alone - - see Benz Dr.'s weigh-in on that one in a post above. Wear of any kind was hard to find anywhere, though somehow there was a parts bill of several hundred dollars all up, mostly for gaskets, seals and external fitments. I left for home and other issues. Joe assembled it -- without the external Tube -- over time as his schedule permitted, even to the inspection paint codes. He filled it with Valvoline synthetic lube, and I doubt very much that he overfilled it: how many has he filled in the last quarter-to half century? I drove it home at Interstate speeds, savoring the 3.46 ratio. At the end of the ~300 mile trip, the rear 15-20% of the car was a slimy mess, under and above. Lube definitely did still remain inside, but little and only discernable in the axle tubes. The differential housing was just comfortably warm and has not made any noises at all, ever, so apparently it was caught in time. I subsequently re-installed The Tube, modifying it to fit, drained and refilled the lube. [Drained fluid was filtered thru coffee filters and yielded zero sediments of any sort to the naked eye.] Beyond a few sessions re-tightening the poor threaded fittings -- tested mightily by the alteration process but proving quite stout -- it has been fine ever since.
The car and our relationship being what it is, actual mileage accrued over the intervening year and a half hasn't amounted to more than a few thousand miles, but did include several hundred- and two-hundred mile outings plus another round trip to Ohio for PUB, which entails two 250+ mile runs at speeds over 80mph in some states.
Michael - Yes, the vent is separate from the U-tube. It's located up high on the central differential case, smallish and hard to see and reach after the whole shebang is installed underneath the car. The Tube, absent in your case, is for circulation of the lube and doesn't directly involve venting of anything. It's purpose may have been for cooling the lube in it's more demanding original duty under much heavier sedans. Ours shouldn't need it, it gets in the way, and it works out fine in most cases to just leave it off.
However, for some unfortunate wretches among us..... Oh, let me put it another way: you live right, don't you? Well, I liked to think I did, too. And then this. I tell you; it just shakes your faith in the fairness of things. As for you and all the others like you, go Tubeless and enjoy, lucky so-and-so's! : )
Denny