Lots of Sienna folks run an XL rated tire. Popular ones are the Falken AT3W and the Cooper Discoverer AT 4S. The Falken AT3 Trail is a little more highway oriented the the AT3W. I've run the AT3W and AT 4S personally and think the Falken is the better tire if you care about snow and dirt performance.
Not sure if airbags are an option for ya. But they made a big difference for me. <$100 and an hour or two to install. 3 years of good service so far. I keep them at ~18# when standard camping setup is in (95% of the time) and bump it up to 25# when adding 2 bikes to the back or packing for a long trip. Keeps the sag away. Obviously won't fix blown shocks though.