You can use the VIN to lookup the specs for any vehicle as it left the factory. The tow package option is the only one that is actually a bargain as you get the hitch, the wiring, and a better radiator core and a transmission fluid cooler in the package for peanuts. They usually include a trailer brake controller and sway control. Ford calls this the "Heavy Duty Trailer Tow Package". With this tow package the maximum trailer weight increases from 6,000 lbs. to 9,000 lbs. with the 4WD version of the Expedition.
Download the Ford towing guide as it specifies by model and engine and gearing, how much of a load the vehicle can tow. You can find this easily to download for all the manufacturers' vehicles.
Take note of how much the gearing affects the towing capacity. Don't make the all to common mistake of getting higher gears that will hurt gas mileage when towing and not provide a noticeable difference in MPG in actual around town driving either. For the Expedition get the 3.73 gears and you will be glad that you did. The lower gears makes for faster acceleration to get onto a freeway with a trailer in tow and for safer passing when pulling a grade with semi's doing 10-20 MPH in the right lane.
The lighter the trailer the more stuff you can carry comfortably. The newer trailers are much lighter though also more expensive. Some like the ones from Lance are a good medium in terms of weight and priced lower than most.