Isolating vibration sources is a tricky science with the best of equipment (better equipment than most dealerships or repair shops will have) without it, you can still apply some science on your own. "Tire/wheel" vibration (you have ruled out at this point) is the most common source in vehicles today; it is relatively low frequency. A single out of round wheel or tire or vibration will "thump" at 10 - 14 Hz on the highway. Not enough to "hear" but, you can "feel" it. A bad set of tires; or, a tire monkey who doesn't know how to use a balance machine will cause more than one thumper and resonances will occur and change as the "thumps" will add and subtract from one another as you maintain speed and follow curves in the road which allow the wheels to index relative to one another. Bad front wheel/tire vibrations will "nibble" at the steering wheel causing it to "twitch" which you can feel when you're holding the wheel. Once again, if both front wheels are causing problems, this "nibble" will come and go as they cancel each other out, or combine their forces through curves and turns.
Flywheel accessories, typically A/C compressors can cause noticeable vibrations. I've noticed that as the A/C systems get older in all of my Fords (for the last 40 years) the amplitude of their vibration increases and becomes more annoying to me. While these accessories are turning around 50% faster than engine speeds, they are complex compressors with multiple orders of vibration exciting in multiple vectors that I believe end up exciting other things, perhaps even the entire engine/transmission sub assembly on the mounts and presenting at a lower frequency than their turning speed. I tend to "feel" it, much like a tire thump. I can tell every time the A/C compressors cycle on and off. This is easy to eliminate as a possible cause, simply turn off the A/C and see if it goes away.
Finally, driveshaft/drivetrain: Most (all?) Expeditions run either a 3.31 or 3.73 final drive ratio. This means that at any vehicle speed, the drive shaft is turning 3.31 or 3.73 times as fast as the wheels. This makes the frequency of a driveshaft/drivetrain vibration at 80 mph ~50 Hz in a superb location in the vehicle to "excite" all sorts of other objects to resonance in the passenger compartment, including your ears. At 50 Hz, the vibration is audible and will sound a little like bass feedback or hum from a guitar amp. The volume will be much less than if you were experiencing the problem in a "live axle" vehicle such as a Pick-up because it can't directly excite the unsprung mass of a rear axle. Vibration can come from any one of several components in the driveline; but, normally once a vehicle is assembled you have to correct the problem on the driveshaft itself whether or not it is the culprit. Additionally, the output shaft of the transmission or transfer case and the rear pinion, must be parallel to prevent vibration from the U-joints flexing out of phase. My experience is that driveshaft misalignments will present at double the frequency of imbalances.
I've had more than 50 new Fords in the last 40 years and I've had to balance driveshafts on probably 6 of them. I've done another 10 or so for friends - Porsche's, Toyota's, Chryslers, Vettes, etc. But, I haven't had to balance one built after 1995. Everyone seems to have gotten better at balancing and assembling RWD drivetrains in the last 30 years.
If you have a drivetrain imbalance, and you aren't experienced with correcting such a problem, you might want to call around to any speed shops in your area and ask if they have equipment/experience correcting driveline vibration (on IRS vehicles). If the dealer or anyone else reflexively tells you that you need a new driveshaft, without showing you signs of physical damage...be wary...there are more than a few threads on this site of people going down that road without success, because the driveshaft component wasn't the root cause.
Good luck and let us know what you find.
