I think there has been some reluctance to suggest the pinion angle - at least on my part - because the vibration developed instead of being there from the start. Also, wouldn't the pinion angle decrease when towing making it even less of an issue?
I'm scratching my head on the dealer's solution. If there was already a hose in place then it is likely that there was no breather vavle present. When the axle valve mod is performed properly the breather valve is removed, replaced with a non-valved fitting and a hose attached. I don't get it.
Referring back to post #12 (mine), my Ranger came back from the shop after a rear axle rebuild (bearings, seals, supposedly ring & pinion). Drivetrain noise is pretty much gone. Vibration at high speed (75 MPH was really high speed for that truck) is gone. Have you or the dealer opened up the differential cover to verify the condition of the gears?
For what it's worth, my axle vent mod has been in place for almost 4 years (August 2007). I checked the filter (located in the tail light) at the end of the hose. It was clean as could be. If it had any discoloration from the hose end I would have been quite concerned as that would indicate that my differential fluid had traveled up 3 feet of hose!