To find the *exact* towing capacity for your specific van, you need to check the sticker located on the driver's side doorjamb. This sticker lists the vehicle's maximum towing capacity.
However, a *general estimate* for a 2000 Ford E-150 with a 5.4L engine would be in the range of around 5,000 to 9,000 lbs. This is a wide range because the options and configuration affect the final number significantly. Don't rely on this estimate for actual towing. Always consult the sticker on your vehicle. Exceeding the listed towing capacity can be dangerous and damage your vehicle.