Weird vibration

I had a similar thing happen with a '72 slant 6 3 speed manual 7 1/4 rear end Duster back in the late '80s.
It turned out to be a rear wheel bearing that was going bad.
Also, it was making a small whirring noise when going around a right corner.
Turns out that it was the driver's side wheel bearing.
A separating tire could make the same noise as well, so i would look for that too
Next time this happens, stop the car after the noise is heard and go touch the tires to see if they are overly hot and put your hand on the axle tube on both sides to see if they are hot on one side or the other.
This is a zero dollar check that might save some time and possibly an accident..............