Tim and others,  The only way to actually KNOW what the axle weights are is
to measure.  Have the van fully loaded with the usual passengers, dogs, cats
(sorry Stan), spares, fuel, water, extra joker in the poker.  Front axle
weight and rear axle weight.  Now you have 'real' data from which to work.
Some of us, loaded for fun, with an extra cylinder or two on the waterboxer
approach 3300lbs measured on the rear axle.

With this data you can see where your PARTICULAR van fits into the spectrum
of available tires.  Then the choice is much easier.  I add a 10%
degradation just for safety and to allow for aging of the tires.

Do remember, brakes stop the wheels, tires stop the car.