OK, here's the whole low down on the rear brake squeal. Toyota first came out with thicker backing plates for the rear brakes. This KIND OF fixed the problem, but not totally. I'm a Toyota mechanic and I have seem hundreds and hundreds of Sienna's with this. The new fix, per Toyota, is to attach stick on wheel weights to the back side of the backing plates. It might seem strange, but it works. I've NEVER seen one come back after this. Either take it to the dealer and have them do it, or let me know if you want to do it yourself and I'll give some directions to do it yourself.