Does that seal ever look overheated? Is it possible the brakes are overheating on that wheel, causing the grease to cook and the seal to melt? You mentioned that tire is wearing more than the other, so it may be possible that wheel is braking more than the rest and overheating.
"Alles hat ein Ende--nur die Wurst, sie hat zwei"
"You only have too much fuel if you're on fire"