As others have said above, you have experienced what we know as the Death Wobble. If you had it at 20 mph (about the usual speed), it will disappear if you speed up. The danger in that is - you have to slow down again some time.
I recommend, like others that you go through the front end of the car and tighten anything loose and repair anything worn. Start with the steering box, then the pitman arm/sector shaft clamp and steering box/chassis bolts, then work your way through it. When I had it, I tightened the steering ball joints a bit - that helped but didn't cure it. I'd pay special attention to the king pins, wheel bearings, ball joint on the A frame, toe in, shackles - heck look at everything. It is quite likely it is a little here and a little there combining to play havoc.