These vans have a much larger frontal and side areas. They have to deal with greater wind loads especially cross winds. How many miles on this van?
. I would check the entire steering system first and replace any worn parts before messing with the steering box,(worm gear), adjustment. The OEM proceedure for box adjustment is very precise and critical. The gears usually wear most in the straight ahead section. Even slightly over adjusting the worm gear will seem OK at first in the straight ahead position but can make the steering bind up in turns when under a load. That can be very dangerous and accelerate wear to the box at the very least.
I'd check the steering column shaft coupler for wear first as this is where most play occurs and replace that as well as any other joint and links as needed. If the wear is in the box and if it's high mileage I'd replace the steering box with a reman unit.