When I went from a 4,400 lbs. TT to a 7,400 TT (yes, still well within limits of Expy, used a Cat scale and tongue scale to balance the load), I felt very unsafe in my Expedition.
The independent rear suspension and passenger car tires made the back end way too wiggly for me. With coil over shock suspension, I couldn't add airbags. I toyed with putting truck tires on, but decided spending more still wasn't going to fix the basic problem of being at the very edge of my payload rating.
You have two more nearly-adult-sized people than I have so you'll be pushing the payload limit even more.
I cannot express how much better towing with a HD pickup is over the Expy. Even with with the lighter trailer, I never felt at ease towing with the Expedition. It's nice not having to use strap on tow mirrors or an add on brake controller, too.
