The all wheel drive version is heavier and more sure-footed than the rear
wheel drive version and has a lower center of gravity so it is not affected
as much by wind.  That said, I drove about a dozen rear wheel drive Previas
and don't recall them being that bad, so there may be some things you can
check.

First, make sure that the tires are inflated to 5 PSI over the recommended
cold tire inflation pressures as indicated on the driver's door jamb or
glove box lid.

While you are looking at the label, make sure that the tire size mounted on
the van is the recommended size and that all four tires are the same brand,
model, and have at least 2/32" inches of tread depth remaining.

Check the rear shock absorbers and front struts to make sure there is no
fluid leaking and that they are not worn.

Have the wheel alignment checked.

 It can be tires.  Some "Passenger Car" tires (tire sizes start with
P) are spongy squishy soft to ride better, but they don't grip the
road worth a darn.  

 On the next windy scary day when you are being tossed all over the
road, pull off and fill the tires up to the max on the sidewall (32 or
35 PSI) or even 2 - 4 PSI higher.  (Don't leave it like this for too
long.)  Now try the same stretch of road again, and see if it helps.

 If you are due for new tires, consider going to a stiffer passenger
car tire that handles better, the tie shop can help you choose
something more appropriate to the kind of driving you do.

 Or go to "Light Truck" (LT prefix) tires that are a lot stiffer, so
the ride will be very harsh compared to passenger tires.  They won't
corner quite as well because it's a harder rubber compound, but when
lightly loaded they should handle better and last a lot longer.

 After that, check the shocks/struts.  Jump on the bumper at each end
(one at a time) and get that corner bouncing hard at least 6" per
jump, then stop - the car should settle down after one, maybe two
oscillations.  Worn shocks or way out-of-balance tires will let the
wheels bounce up and down while driving, and all the time the tires
are bouncing in the air they can't grip the road.

 Then it starts getting more esoteric - like front wheel or four-
wheel alignment.  There are usually a range of "acceptable" alignment
settings, but they need to be closer to one end or the other of that
window to handle best.  A good front-end alignment man adjusts for the
proper end of the settings to get the best handling.

 --<< Bruce >>--