Depends on the load, the speed, the wind and the mileage on the shock absorbers. First inspect your suspension and make sure your sway bars are all ok. If you load this van, then I would go to heavy duty shock, I have had good luck with the Monroe Load Leveler series and if needed go to a heavier sway bar set up. FED EX does NOT use stock parts if you get my drift.
I had a Maxi Van. The fuel tank was behind the rear axle, taboo. When I filled the tank the van would sway, excessive weight behind the rear axle. I had it removed and installed a tank infront of the rear axle and had 4 heavy duty truck tires install. It did not sway again.