One option you will not like, drive slow, like 55-60 mph and the other is to raise the rear gearing to 3.73's from the 4.10's. I wouldn't go any higher due to weight of the vehicle.
Another option as was suggested before is a good air-dam and if you have the time and money a full belly pan under the vehicle.
The largest problem you have is the size of the van. There is no easy or cheap way to drop the Cd and or weight on that beast.
