So, let's see. You took the truck to this guy to replace your front axle seals. He does the repair, you pay him and take the truck. Two days later the seals are leaking worse than when you took it to him and he says, "Oh yeah, that RH shaft is nicked up pretty good. You'll need to replace it to stop the leak." and you consider him a "good reputable mechanic"?
Sorry, but that pig wouldn't fly at any shop I've ever worked at. If the shaft needed replacing he should have notified you before the repair, not just put it back together anyway. My initial feeling here is that he had a hard time with this repair and knew he didn't get it right, gave it to you anyway HOPING it would be OK and now he's trying to divert the cause of the leak to bad parts instead of a poor repair. Even if the shaft is bad, it's still on him for not notifying you before-hand.