There is a difference between what it CAN do and what it is DESIGNED to do. Sure your Expedition has the power to tow 6000lbs, and you can probably get away with if you only do it once or twice, but if you do it regularly, your drivetrain will quickly wear out and break faster because you are overstressing the components that is NOT designed to tow that much weight, like the Engine, Transmission, and Rear Ends. Keep in mind, that your Expy already weighs close to 6,000lbs by itself, add to that a 6,000lb or a 9,000lb trailer, that a lot of weigh you are trying to pull. You could probably tow 6,000 lbs with a Volkswagon, if you tried, but it would start falling apart almost immediately because it's not designed to tow anywhere near that.