I have 2005 Ford Explorer XLT AdvanceTrac that is all of a sudden giving me a front end vibration problem that is at it's worst right at about 70MPH, less so 3-4 MPH either side of that. The vibration may be coming from the driver's side. I say that only because the driver's outside mirror vibrates at that speed too, where the passenger side doesn't.
I had the wheels balanced by a local tire store, and it didn't help. I haven't taken it to the dealer yet, anticipating a run around, thought I would check with an expert like you first for your thoughts.
If your Explorer is a 4x4, Ford Technical Service Bulletin 05-16-4 deals with what they refer to as a vibration/drone/boom at 55-75 MPH. This condition is experienced only in 3rd gear with overdrive off, and is not felt through the steering wheel, more so in the seat of your pants. The TSB covers 2002-2005 Explorer, Aviator and Mountaineer, and the fix is the installation of a "premium balanced" driveshaft (whatever that means).
If you have a 4x2 explorer and the vibration is felt in the steering wheel, that usually indicates it's coming from the wheels and/or tires. And just because wheels balance correctly, does not guarantee they will not cause a vibration or shimmy, especially at high speeds. There may still be excessive wheel and tire radial run-out. The dealership has special equipment to measure and correct radial run-out. It involves breaking the bead and rotating the tire to a position on the wheel where the run-out of each will counteract each other. This is referred to as match mounting.
Another common vibration complaint is due to flat spots on the tires caused by the vehicle sitting for long periods of time. It can take up to 200 miles of driving to smooth out the flat spots and eliminate the vibration, but this is considered normal.
If the problem is not felt in the steering wheel, and is a true driveline vibration, it can be caused by a loose u-joint, unbalanced driveshaft or improper driveline angles. The techs at the dealership can also evaluate a driveline issue if it comes to that.