I've had a similar issue with this myself when I first started towing with my wagon. I had no idea what I was getting into at the time. Based on brochure weights and what I thought I was packing, I estimated I was towing 4-5k lbs in a light weight 26' trailer (bumper to ball). I didn't understand the effects the length of the trailer when impacted by wind or other vehicles would have on my stability before hand. Test tows on back roads were fine but the first camping trip had us on the highway immediately and I was struggling to keep it in my lane. I tightened up the friction sway control bar and that helped considerably and I was able to make the trip but I was not happy with how it behaved. To reduce the side to side motions the bar was so tight I could feel that it was not letting me just travel straight down the road as well, I was constantly making small corrections. It was easier to drive around curves and maneuver in traffic, it just didn't like going straight and still was impacted by large vehicles to a noticeable degree. I then upgraded my sway control to the Reese Dual Cam system, that made a HUGE difference. I then felt like it tracked straight down the road, even large semi's passing me rarely moved me around unless we both were hugging the center line of the interstate lanes. I still had some issues on windy days. A couple of years later I upgraded to a performance summer tires designed for SUV's. That improved things even more and we had a really enjoyable drive from MI down to the Smokey Mountains last year with no issues. One very windy section had me slowing down to 60-65, other than that we cruised at 65 the entire time. Since then I've taken a fairly long trip with a borrowed F350 supercab long box and a shorter trip with my newly acquired 2500 Suburban. Those both seem better yet. Very stable.
To compare my experience with your situation, I see some differences. First, I never had stability issues related to speed other than the fact that if the wind was blowing me around or a semi was blowing me around, going slower made it easier to handle. On back roads up to 60 mph with no traffic passing me (in the same direction) I felt very stable other than the issue with it not wanting to track perfectly straight, that was more annoying than worrisome. If with no wind and no other vehicle around you feel unstable over 55, then something seems wrong to me. It might be helpful to get your vehicle and vehicle trailer combination weighed. With this you can determine if you are overweight, what your tongue weight is, if any particular axles are overloaded and if your W/D hitch is set up correctly. If your tongue weight is low that can lead to instability at higher speeds, definitely something I would want to understand if I were you. At one point I took the time to weight my set-up and found that my trailer was much heavier than I thought (nearly 5600 lbs compared to my 4-5k lb estimate) and my tongue weight was 970 lbs, 17%. Much higher than the 10-15% recommended. For RV's many suggest getting down in the 10-12 range can be less stable, most shoot for 13-15%.
There is another forum member on here that tows a longer trailer than yours that is around 6500 lbs and he feels he is at the absolute limit of his Expedition. His rear axle is nearly at its limit. If you weighed your rig you might find you are overloaded and it may make sense to upgrade even if your issues can be corrected with the Expedition.
Another difference between the options I had versus you is my wheel/tire combination happens to be the same size as some older Porsche SUV's and I had a nice XL high performance tire option to go with which gave me a much more solid sidewall in my tire with tread that had next to zero squirm. A huge difference. For you, you might find a LT truck tire, versus a P metric tire, would be beneficial for stability.
You didn't say what kind of W/D hitch you had or the type of sway control. You might find significant improvements there as well like I did. First I would weigh everything and see what your needs are as far as addressing potential tongue weight issues and the towing and weight capacity needs of your tow vehicle. I think it is necessary information to have regardless if you keep your Expedition or buy something different. You do not want to buy another tow vehicle and end up in the same situation.
As for 2500HD trucks being gas hogs. The newer ones with the 6-speeds transmissions are much more fuel efficient than the older ones with the 4-speeds. They are probably as fuel efficient as your Expedition. New body style pick-ups starting in 2007 model year have 6-speeds (2007 Classics are the old style body and 4-speeds). 2008 was the first year for 2500 Suburbans to have the 6-speeds as the first year of the new body style, 2007, still had the 4-speed trans.
2014 Wildwood 26TBSS - Upgraded with 5200lb axles and larger Goodyear ST tires
2003 Chevrolet 2500 4x4 Suburban 8.1L 4.10's
