Wow. I have to say after reading this entire thread this morning. This MY specific ('24) Expedition/Navigator highly specific speed vibration (73-85 mph) issue is likely a build problem, and knowing how the truck is put together, most likely a pinion/transfer case alignment issue. I come to that conclusion with ~60% certainty. Of those '24 owners experiencing this vibration, how many are SWB and how many are LWB (MAX's)? If it is confined to one WB or the other, I would change my armchair diagnosis to the driveshaft (which is unique).
As for the whole Ford doesn't care if it's over 73 mph BS, remind them that many states (Michigan among them) have speed limits in excess of 70 mph. Furthermore, though I don't see a lot of Expy interceptors, there are thousands of Explorer rear/AWD interceptors out there and I pity the fool service rep that tells a local Sheriff or State Police Captain, that Ford doesn't worry about vibration over 70 mph, LOL! Additionally, many of KTP's Expeditions and Navigators are sold in the GCC (Arabian Peninsula area countries) where these vehicles are driven flat out all of the time. As others have attested, I can confirm that my '20 MAX has managed to see 3 digits more times than I can count with no identifiable resonance (while wearing Defenders) and actually seems to smooth out, presumably because the air we breath is supporting several hundred pounds of vehicle weight that typically rides on the tires at lower speeds. If people on this forum are noticing this, trust me, the plant is getting complaints from Police and GCC customers. Heaven forbid, some misguided soul came up with a procedure or fix for those specific customers and ignored the rest of you.
If in fact it is a drivetrain alignment issue, the universal joint bearings are at risk, especially when you're generating enough resonance to shake the seat. There is your safety issue. Nothing worse than driving at a legal speed (80 mph in SD, for instance) and having a universal joint let go. In fact, if alignment is an issue and you've experienced the vibration, when they fix the issue, have them closely inspect the U-joints.
30 years ago, we used laser alignment tools to check T-Bird and Mark VIII drivetrain alignment (diagnostically), I'm sure that somewhere in Dearborn they have that (or better) capability today to check full size and midsize SUV's drive line alignment. It can be done with steel rules and protractors as well by people who know what they're doing (It's been at least 35 years since I've done it personally).
Finally, the other 40%...There could be an common imbalance in the rotating components such as the transfer case output shaft, driveshaft, or Pinion. If this turns out to be the case, the solution should be to dynamically balance the drive train by adding weight to the driveshaft on a vehicle already built, this is easier than it sounds....but cannot correct an alignment issue.