Serious steering wheel wobble

my 69 Mach1, has a seriously violent steering wobble at about 70mph up to 80 mph, then quiets down from there on.

I run stock wheels with BFgoodrich 215/60-14 all the way around.

Does anybody have an idea of the cause?

Tire balance? I've had all 4 tires re-balanced
Wheel out of round? (no clue where/how to test this.)

Drive Shaft out of Balance?

Bad ball joints? ( wheels wear perfectly even, and the car runs straight with no pulling, but I believe the joints are original)


The wobble is best described as the whole steering column girating up and down rather than just the steering wheel having the shakes.

Thanks

Chris

The column is secure in it place...the u-joints are relatively new ( < 500miles).

I'm going to try to get the drive shaft balanced, but I seem to remember out -of-balance drive shafts were felt through the entire car in a harmonic (sine wave...at a certain speed the car would vibrate then stop then vibrate several times per minute in a pattern) way.

any other clues?