The mirror reference is only a clue. In my 84, the rear view mirror would start to vibrate vertically at between 25 and 30 MPH and increased in severity as my truck rolled faster. I was able to isolate the possibilities based on the direction the mirror vibrated and the characteristics of the vibrating mirror. As I watched it, I noted that it vibrated consistently with the speed of the wheels and not of the reciprocating mass of the engine. It also did not vary with the direction I turned. In the end, I determined that the vibration had to be isolated to the rear axle. I took them out and had them spun on a lathe. That's where we found the wobble. It was so slight that a visual slow roll inspection could not discern it. But a fast spin and a trueness gauge spotted it immediately.
Just a thought, although I think by sheer elimination, you are already there.