I noticed a few other expedition owners posted in here but didn't mention one thing, your tires. I'm not sure what options your 15 has, my 13 came with crappy P metric tires that were about at their max before I put anything inside the truck. My Expedition weights in at 6,100 lbs without a driver. My best advice before you go jumping off a bridge or buying a used excursion (I had one and hated it on ever level of crappy from towing to ride to braking to non supportive seats) try a set of load range E tires on your truck. When I moved to the new expedition from the old one I was reminded quickly just how bad P metric tires are when towing and in general. You will see a night and day difference running some good tires on that expedition even driving around town, turn in will be quicker, you will feel less roll when turning since your outside tire will not flex as much under load, overall your experience will improve.
I am running 285/55R20 Michelin LTX AT2s on mine, 50 PSI empty and 75 PSI if I've got a heavy trailer.
I would also makes sure you are running your trailer tires at the best pressure possible and that they are all at the same pressure, this all contributes to instability. While you have a huge trailer as far as length I don't see a major weight issue by any stretch.
On a side note I assume you mean you have a standard length expedition not the EL correct?