1998 Toyota Sienna thumping noise when I brake

1998 Toyota Sienna mileage: 50,000. As I brake, I hear a thumping noise from the rear. The thumping slows as the car slows. Mechanics at my dealer told me it was my *front* rotors needing to be replaced. That didn't sound at all right, so I took it to an independent mechanic for a second opinion. He said that the rear wheels and brakes were full of dirt and gunk (and that the front rotors were fine, thank you very much). They cleaned it all out and the thumping went away -- for about a week. Any ideas?