my rear brakes seem to have frozen so the rear wheels wont turn.

I have the same problem with my 2013 Sienna XLE. Every time the van is washed in winter the brakes freeze up & the rear wheels are locked. We live in Western New York. When I returned the van to the dealer, I was told it can't be washed when the temp is below 32 degrees. They refused to look at it even though it is under warranty.