had same issue (though wasnt new when i got it) but found dust cap had a ping/dent/ or whatever you want to call it where it mates to the hub allowing grease to seep out at high speeds. picked up two new caps from CW and fixed issue. you might spend more in gas towing to a dealer and having to wait etc for them to do the same or just get a new dust cap for a few bucks. I picked up 2 (sold in a pair) with the rubber plug for $6 out the door. just a suggestion.
*Currently trailer-less but shopping
07' GMC Envoy w/ P2 controller (current TV)
88' Chevy K5 Blazer (soon-to-be TV)
74' VW Superbeetle (1303)