Hmmm, Violent shaking is definetly not much fun. Fortunatly for you, it's pretty easy to diagnose and some of the repairs can be made by a weekend warrior.
First, you need to check your tires for inflation, cracking, seperation and most importantly balance. If you have a single tire up front loose a balancing weight, it thows off the whole geometry of the steering. At a certain speed you come to harmonic flux, which is the speed at which an unbalanced tire resonates (frequency) and creates friction and viola', shaking. Since most people never really maintain their tires, it'll probably save you money from now on to have your tires rotated twice a year and when the tire shop rotates them, have them balanced. This if free for people that buy tires from reputable dealers.
Now, if your tires are all in good shape, properly inflated and balanced and there are no signs of seperation, I would then look at the drag link, and it's components (inner/outer tie rod and ends) if anything looks bent, it's time to have a mechanic continue the work. If everything looks straight, give the tie rod ends a good tug in all directions, if they wiggle, they're bad and need to be replaced. While you can do it yourself, you will need an alignment when finished.
While I'm on the subject of shaking, it's important for you to understand that shaking is an alignment issue. There is a part of the vehicle or tire causing the wheels to spin out of alignment and that causes the different frequency that causes shaking.
Anyway, if your tie rod ends are good, your drag link is good, your tires are good, your original alignment is good, then it's time to check for wheelbearing freeplay. Lift the wheel at the axle, shake it, if it wiggles but the ball joints don't move, replace the wheelbearings, if the ball joints are wiggling, then replace them.
In end, heres a list of things that could cause this, and it's a pretty short list, I'd suggest learing tricks to test each and take them in a logical order.

Alignment
Tires
Steering components (tie rods/drag link) BTW GREASE THESE OFTEN!!
Upper/Lower ball joints
Wheel bearings

The good thing is that if you were to have an afternoon and a friend that's mechanically inclined, these are all things you can do yourself, even the alignment but you need a really smart friend for that one. The prices for the wheelbearings are cheap as ever, inners on both sides are $10 ea, outers are $12 ea.
Drag link, tie rod ends etc are cheap too, about $80 to replace the whole deal.
Upper/lower ball joints are pretty cheap too and you can replace them all for about $100 yourself but you'll need to have alot of faith in your mechanical ability for these, they suck on Bronco's and are a BIATCH for a nonpro especially the first time.

Well, thats it. Hope this helps someone.

Chaiyz