High top conversion van Handling in the wind

The first thing I would look at are the tires and it sounds like you have done that. I hope you installed light truck tires NOT car tires. Years ago when I got my first van, the previous owner had installed car tires. I suppose that seemed reasonable since they came in the correct size and were inexpensive, but the handling was terrible. I could not believe the difference when I installed proper tires. After eliminating (or correcting) tires as the source of the handling issues, I would take a look at adding some kind of rear suspension enhancement. I have used helper leaf springs which really did help, but better yet is a rear air bag system which helps more. Finally, you might consider a "Safe T Plus" type steering control system which really amounts to a large shock absorber that connects to and stabilizes the steering system. I have one on my Dodge Leisure Travel Van and it makes a difference. Beyond all of this, high profile vehicles are inherently going to be more susceptible to side wind loads than other vehicles. Driving slower in windy conditions is usually required.