I had the same problem with my '94 Explorer.  It turned out to be a bad
speedometer, and I replaced it with one from a junk yard. (I wasted a
lot of time lubing the speedo cable etc before deciding it had to be
the speedo itself. The key is the HIGHER than normal speedo reading.)

I took the bad one apart and figured out what caused it. The '94 and
earlier speedos are driven mechanically by a cable. The cable spins a
magnet in the speedo. The needle is attached to a metal cup that fits
over the magnet. To keep the cup centered over the magnet there is a
bushing in the center of the magnet assy, and a mating pin on the
speedo needle assy.

When the bushing wears, the metal cup attached to the speedo needle can
flop around. If the magnet rubs against the cup, it causes the speedo
to indicate a HIGHER speed than actual. In my case, the needle tended
to swing wildly too as it randomly touched the magnet more or less.
Like yours, it only happened at higher speeds, 55 mph and up in my
case.

If you get a replacement from a junk yard as I did ($17 at pick and
pull) check the side play on the needle and compare it with yours. The
new one should have very little side play compared with the bad one.
This is a common problem, so you might see it on other vehicles with
high mileage.

I got a speedo from an old Bronco, a 94 or earlier Ranger should also
work if you can't find a donor Explorer.

If you have a 95 or later Explorer, none of this applies. Those have
electronic speedos that work like tachometers- they use the pulses from
the VSS (vehicle speed sensor) to calculate the speed instead of a
mechanical cable.
-Paul

It's also possible it is just the cable needing grease.   You might
want to try that first before you take the whole dash apart to replace
the head.