Well, now that you say it's shaking at 30mph, I'd say check those wheelbearings. They fail often on Bronco's and especially on Bronco's in the snow with plows. If you were to jack a tire from the axle, and shake it vigoriously, you'd know if it was those bearings. It'll feel the same as a bad ball joint, but the ball joints won't be moving, you might need a friend to shake it while you start at those balljoints. If there is any play or wobble and it isn't the balljoints, replace the wheel bearings immediatly.
One of the main reasons I know so much about this subject, is I had a passenger side outer bearing fail going 80Mph on the highway. Not only did I ruin my hub, rotors, calipers, spindles, axles etc. But the tire shook so bad that it blew, and took out the entire fender and peeled back the door skin about 4".
It's the scariest thing that's ever happened to me on the road and It's something that has the potential of causing death, not only to you but your passenger and anyone else on the road next to you when the bearing fails.
Replacing the bearings is pretty easy, just remove the hub and it's just like on a 2wd vehicle. Removing the hub assys. is a pain the first time but if you take your time, you'll be fine. Anyway, let me know, I've very curious about this.
BTW, unless it's completly frozen, it's not the steering stab. I've heard of them getting very loose and causing a loud rattle but never heard or seen one cause serious vibration in the past 20+ years.

Chaiyz