You couldn't be more wrong. The old style breather causes the seals on the diff to leak, it doesn't cause the diff to fail. The new style breather is only usually installed when repairing a leak or replacing the diff.
Look, not going to start a pissing match here, but let's talk what happens when breather gets blocked. If it causes seals to leak, thats from overpressure, IE pressure cannot equalize through breather. Oil leaks aside, if pressure cannot escape, then certainly moisture cannot get out of the diff as fluid heats up to op-temp. Moisture doesn't get out, it starts working on bearings, then at some point you've got a bad diff on your hands. at the very least the two outside bearings that make tons of noise. Thank god the things don't just grenade, but you've still got moisture that is only going to build if the membrane on the breather is blocked. If someone got super lucky the leaky seals would allow moisture to escape but that is far from ideal.
You're totally entitled to your opinion but actually seeing the difference firsthand is what leads me to believe this is the case. If that's the practice dealers went with, we all know how that's gone. People have bad diffs because the manufacturer made it a 75k service item and not a 30-50k one. I'd say that's a pretty widely accepted reality. In theory if they had good breathers they might have gone much much longer before experiencing the current crop of "bad" diffs.
Repeating what your dealer says to a guy who is looking for a used truck with more miles is just a recipe for an empty wallet in the future. IMO go with diffs that had the TSB done.
