Wheel tru, bearing & cones tight but still WOBBLES
As one commentator pointed out, the frame was actually cracked (severed even) at the end of the chain stay (on the gear side) near where the wheel is mounted.

For some reason I only noticed it tonight after riding home from work and experiencing an even worse wobble. Perhaps the crack got worse and became through-and-through on my long commute today.

While frustrating, I guess I am lucky I didn't get hurt...

Thanks for all the helpful suggestions...

One question, is it possible (and worthwhile) to have someone weld it back together -- while the bike is almost 40 years old, the frame is solid, heavy and steel (it's a Trek Mountain Bike from the early 1980's).