I totally hear what your saying, it is an in and out droning vib felt thru the car after 3500 rpm starting at 70 and up.
On long drives where the speed limit is in the seventies it gets to be a real drag.
I went thru my dart from front to rear all mounts complete front end rebuilt with bushings. Checked pinion angle, tires, new drive shaft after ballancing three.
On this next build I have had the engine ballanced at a different shop hoping that might be the thing this time.
I feel this same vib in lots of mopars from pick ups, vans, 4 speeds, autos, it seems to be small blocks from what I have noticed and cast cranks.
mmmm, maybe something there.