I have been extremely impressed with my service rep at Park Place Lexus in Grapevine, TX (Abe is fantastic) - I had this issue at about 7500 miles and he just took care of it - no questions - no nothing.
Excellent customer service.
I had the same problem - my lexus service rep said it's a common defect - especially while backing up. The dealer resurfaced the rear rotors and replaced the brakes front and back under warranty. Hope this helps.
Doug