There is a rather inexpensive <$100 air-lift bladder available (I believe Firestone is the actual mfg.) that fits inside the Sienna rear springs and seems to be very popular with the Sienna towing crowd. It keeps the rear-end from sagging with a load.
People who have installed it suggest it's about a 2-hour DIY job or a <$100 install job from your local tire dealer (e.g., Les Schwab).
I suggest checking out
www.siennaclub.org for more postings on the subject.
If you have an AWD Sienna with run-flats, you might want to reconsider using anything smaller than a 2" receiver. If, like a lot of Sienna AWD owners, you come to the realization that the run-flat tires are over-priced, rapidly wearing, less-than-ideal tires, you are going to want to figure out where to put a spare.
And if that's the case -- and you don't want to down-grade to smaller wheels and tires (so that one can fit inside a 3rd-row seat well -- for the price of one run-flat tire, you could hook up with a Bonarue 2 Spare Tire Carrier-like device (which only fits 2" receivers). The main thing here is that it easily flips down so you can open the rear hatch door.