We were there two weeks ago with a Toyota Tacoma. There is one short section that you'll want high clearance for. It's a short, bumpy hill. We threw it in 4WD for kicks, but that wasn't strictly necessary. I don't know exactly what a Sienna is like but if you take it slowly, you might be okay. Scout it out, if you don't like it, then you can walk the remaining distance to the crag. I calculate that this would add 2 miles to the approach.