I have a 69 Bronco all original basic V8. When I hit an abrupt bump on a road at 35+ mph especially if it hits both wheels at the same time like a bridge, both front wheels violently bounce until I hit the brakes and slow it to 30 mph then it goes along fine. The steering wheel goes crazy when this happens. There is also a bit of slop in the steering but after driving this thing for 20 years I'm used to that. I have new front shocks and everything seems tight in the tie rods ect, the steering stabilizer seems to be loose when I shake the actuator rod. Other than that, drives straight down the road as long as I hit the brakes before I cross any bridges or hot big potholes.
Sounds like death wobble
Sounds like death wobble to me, I would check all you steering linkage for slop. Also check the holes in the steering knuckles, where the tie rod ends connect. They may be worn so that they're out of round. My dad had a '71 f100 with the same steering setup, with a lot of slop, one day it just broke and no steering, fortunatly he was only going about 10 mph.
This guy has a video on his website that shows a death wobble pretty well.
http://www.emotoman.com/videos/video.html
Re Buckin Bronco
NO DEATH WOBBLE! It was a bit hairy. Turns out that the steering stabilizer shock was wasted and the drag link bushings had slop. With a fresh alignment I think this front end is good for another 39 years.