Haunted maybe?
I'm sure GPatrick will provide some input to this post as, although I haven't changed the quantity of components you have, I have some slight vibration issues that I haven't resolved.

Have you ever changed the front spindles and or the front hubs?    I think I'd be inclined to try replacing the front hubs one side at a time if it bugs you that much.   What have you got to lose?  Its only $$$. right?

Steve, you used the word "Harmonic" that leads my feeble mind to something is getting excited at a frequency that has nothing to do (necessarily) with the rotating parts, but gets it thing on at that beat! My experience is that all kind of stuff can get in the beat, but most usually the whole engine tranny assembly gets to moving in harmony with the speed of things. If the problem is for your ears more than your bottom, it may be a panel somewhere getting going. If it is your bottom and or hands on the wheel noticing it, most likely a drive train issue. I had to add about 8 pounds to the tail shaft housing on one car to get the thing to quit the vibration. I tried different motor and trans mounts, but it only quieted down with the balance weight on the tail shaft housing hanging out about 10 inches. Mass of engine tranny assembly/ engine and tranny mount "K" factor, excitation forces all equaled a shake at a frequency. I also had the problem with the floor pan once that did a similar thing when a certain speed was present. Ended up  putting an additional rib in the floor (BFH). Noticed that one when the kids were in the car and it didn't happen. Had the kids lift their feet and vibration back. That one wasn't a Mustang though it had a lot larger floor pan.
Just more food for thought!

Dave

Always at 72 MPH or does the problem speed change with tire diameters or gear ratios?  Does it happen if you run it up to 80 and coast through it or only when dwelling at that speed?  Any difference in 4th gear versus 5th gear?  Something you feel or hear or both?  Is it more noticeable with windows up or down?  Gaba was able to get me a recording a while back from his phone that I was able to analyze.  I would need to know engine RPM, what gear you are in, and tire diameters to calculate forcing frequencies at 72 MPH.

What you are describing sounds like a beat where you have two closely spaced frequency sources that go in and out of phase.  When in-phase they add together increasing the amplitude and later they subtract from one another.  The difference between the two frequencies is the beat frequency.  You can hear this on a prop plane with twin engines when the speeds differ between the two.  What you hear in plane you can feel in a car.  If you can hold steady at this speed, count the number of beats you hear/feel in one minute.  You might want someone else look at a watch while doing this.  The beat frequency can help identify the two frequencies in play.

While sitting in the car, you could have someone impact some of the panels on the car with the heel of their hand to see if any of the natural frequencies of the various panels match the sound(s) you are hearing.  Try several sections of the roof, the hatch, rear window, etc.  A little vibration source can excite the natural frequency of a panel and if the source and natural frequency don't match exactly you can have a beat.  For example, does the beat that you feel/hear change when at 71 or 73?  If you have an hour to spare on the bash, I can bring an analyzer with me to do some basic tests to see if anything stands out.  If impacting the rear window matches the frequency we can measure the actual frequency and compare it to the forcing frequencies of the engine and driveline.  By the way, you don't need wail on the panels to excite their frequencies so don' worry about dents!

In the short-term, a audio recording would be a good starting point.  I can import most .WAV files and Gaba knows the technique.

Hot Rod did an article on a vibration problem few years ago on a 67 fastback. Maybe it will give an idea how to approach the problem.

http://www.hotrod.com/how-to/chassis-suspension/1405-1967-ford-mustang-vibrates-during-right-turns/

David67 wrote:

Hot Rod did an article on a vibration problem few years ago on a 67 fastback. Maybe it will give an idea how to approach the problem.

http://www.hotrod.com/how-to/chassis-suspension/1405-1967-ford-mustang-vibrates-during-right-turns/

Yes, that is a very good article on tracking down a vibration. 

I would call an Exorcist...jj

Sounds as though you have changed the usual suspects multiple times, only leaving a body panel as flexing in the wind possibly. 
Have you  put the rear on some safe stands and run it through the gears to see if you get the problem then?  You say problem started in 1993 or that's when you got the car and it's done every since you have owned it?  Or did you change something in 93?

Well, not fanbelt related.
Vibration goes away if clutch is depressed at speed. Car becomes smooth as glass at 80 with clutch pushed in.

MS wrote:

Well, not fanbelt related.
Vibration goes away if clutch is depressed at speed. Car becomes smooth as glass at 80 with clutch pushed in.

 
I had a similar problem with my 66 coupe. When I got under the car to check the pinion angle I found that the axle had shifted slightly and was actually crooked from side to side. Apparently the u bolts were not tight enough and the resulting movement wore the aluminum lowering blocks and every thing shift catty wampus. Could be that for what ever reason the axle is not square in the car in relation to the drive line. Does any of that make sense?

Mark Z wrote:

Put it on a DIno and walk around the car and look what starts to vibrate as they run it.

Good Idea!
 

Car has been driven across the USA with varying amounts of passengers and cargo in the trunk.  No change.

I just drove it with no AC belt.  Made zero difference.  Did some testing with varying scenarios.  In fourth gear at speed, the harmonic vibration goes away.  This is at 75 MPH at about 3.400 RPM.  Or at least is not noticeable at all.

Rear end just replaced with all new stuff 100%.  Is square in the car and properly attached.

So...
Goes away if clutch pedal pushed in.
Goes away in fourth gear 75 MPH cruise.

Thinking of clamping some vise-grips to the tailpipes or something to see if that affects it.  It has done the same thing with 3 different exhaust systems though, all with different sizes of pipe and different hangers.