From a 2011 order guide:
Heavy-Duty Trailer Tow Package
— Class IV Trailer Hitch receiver
— Wiring harness w/4 & 7 pin connector
— Heavy-duty auxiliary transmission oil cooler
— Heavy-duty radiator
— Electronic braking wiring kit
If you plan on towing anything over about 3000 pounds (some states require over 2000 pounds), then you need an electric brake controller which is aftermarket for all Expeditions. There is some vehicle wiring required also that appears to be included with the HD trailing package only, but I'm not sure that is true. I suspect any example that has the 7-pin wiring in the rear bumper has the wiring under the dash as well.
The other big impact to towing capacity for automatics is the large capacity transmission cooler. I highly recommend it for any high walled trailer. They typically cost under $100 plus installation which any transmission shop can do for under $300.
