| This may cost you some bucks but I think you'll be a lot happier in the end. Toyota basically fired the manufacture who supplied the brake parts for your '01 Sienna. When they designed the '04 model, it was a whole new set up and a lot better brakes.
Here's what you need. Both rear drums have been updated to a completely new part. The wheel cylinders tend to leak behind the boots, so get them checked closely too. If you haven't had the rear shoes done yet, I highly recommend those with the new drums.
The budget way is to have the rear brakes cleaned and adjusted, then apply some BG anti-squeal (most dealers have this) to the shoe surface. Then, I affix 1.5 oz stick on wheel weights to the rear of the backing plate to dampen the squeal vibration.
Yes, this is all coming from the REAR brakes, but on occation I see the front caliper slide pins seize and cause a sinking brake pedal feel.
Good luck. | 
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