"So why, invariably do the tire inflation specs have higher pressure to
the REAR on Vanagons?"

my $0.02
It's mostly dynamics because of the effects of momentum and inertia on the
weight of the van while moving, along its length. Although the front
sidewalls "roll" when the van turns, because of lateral oversteer
characteristics inherent in that, the rear sidewalls have more lateral force
on them than the fronts. Higher air pressure stiffens the sidewall to better
deal with that force.

bob