I will mention here what I started thinking about.   I read an article about brakes and brake drums and how auto manufacturers used to balance drums.  It was about new aftermarket parts no longer going through this step.  So that got me to thinking...  What about that big notch that Ford cut into the front brake hubs?  Wondering if this may be causing balance issues at greater speeds.  I started researching, I ran across a cj pony video where they change out some front brake drums and I noticed that on the drum, the big azz balance weight on the drum is directly where the notch is.

TKOPerformance wrote:

Very hard to find anymore, but there used to be shops that would balance tires on the vehicle.  This way they took into account anything out of balance in the brakes, hub, etc. instead of just the tire and wheel.  My farther-in-law swears by this method. 

Yup. Until the old cat retired, we had a shop like this in town. He also shaved the treads to get a round tire. No mas.

TKOPerformance wrote:

Very hard to find anymore, but there used to be shops that would balance tires on the vehicle.  This way they took into account anything out of balance in the brakes, hub, etc. instead of just the tire and wheel.  My farther-in-law swears by this method. 

I had an old guy that used to do that on my cars till he died at his shop. It's not as bad as it seems. He loved that place and was happy to the end. I never found out the name or what kind of balancing that was. Does anyone know? The computer balancing is nowhere near good as that method.

TKOPerformance wrote:

Rufus68 wrote:

If you balance the tires installed on the car, then when you rotate the tires you would need them re-balanced.  That would be a down side to this method of balancing.

Yes, that is true, but I think a small price to pay for the added smoothness. 

This is a fact.  I had my tires balanced on the car years ago (front discs, rear drum).  Some time later I did a brake job and didn't take note of how the wheels were installed, car badly vibrated afterward, had to get all four re-balanced.  I also haven't found any shop that does this anymore.

That went off the rails quickly. 
Every single part (yes, every part) has been replaced at least three times

It is not s vibration, but rather a harmonic above 74mph.   You don’t feel it.   You hear it.

Did this occur without sub frame connectors?

Greg B wrote:

All the same, it's probably time somebody redesigned that front hub.   If they do, you will have one more thing to change out.  I have a set of v8 spindles on my shelf with drum brakes still on them that came off that old rusty coupe I once had.  Both have big counterweights on the drum.  I thought about pulling one off yesterday just to see if it was directly over the notch.

And I know this whole thing seems a bit out in left field, but since the notch is in both sides, and the newer rotors and tires are balanced, wouldn't that make the imbalance in both hubs, and attached rotating masses relatively equal on both sides?   There is your tuning fork... kind of like an exhaust drone.

 
Note that six cylinder drum hubs are not even symmetrical.  I think that notch had more to do with controlling warpage from thermal expansion cycles than balance. It is so close to center, and only rotating about 800 rpm that I doubt it was measurable.

I will be exploring the removal of interior quarter panels that have heavy speakers in them as my next attempt to find the problem.  Just need an appropriate round-tuit to get started. Those panels have never changed since 1992. Only thing not changed.  Vibe is same before and after subframe connectors.

My newer exhaust system has these hangers installed now.
Seem to work well, also believe they transfer more vibrations to body.

MS ...do you still have your hook tabs at rear for easy exhaust removal?
I like the design, thought it may be a source for vibration at speed.
Don’t recall the details on your bracket/hanger arrangement for tailpipes.

MS have you thought about having someone ride in the back seat pressing around to see if a source can be pin pointed before you take the interior apart?

Having the rear seat UP or.......not up....make any difference?
6s6