I did the same thing --it's just easier to replace it yourself than to play the warranty game.  I did notice that the replacement axle had a different boot design--one extra fold to the bellow.  I am hoping it will last longer than the original which all seem to tear near the clamp on 
Agree.  I was beyond the power train warranty so tried to use ESP, but was denied.  Said it was a wear item and because I had aftermarket springs installed. Under my breath I told them to GFY. Bought the factory part, installed in about an hour.  No leaving the car at the dealer, and I know it was done right.