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brakes, alignment, or tire balance... most likely brakes

Let's not forget the tie rod ends and drag links. If these ball joints are severely worn that creates slack in the steering mechanisms. They can fall apart while driving down the road resulting in highly dangerous situations. You may run off the road or worse head on into oncoming traffic. You say 31,000 miles so I highly doubt this is the problem but you never know.

If I had to guess I'd say something has happened to your alignment and the toe-in is off badly.

If your rotors where very hot and you ran through water then they may be warped.( New rotors)

If you ride your brake pedal then it's worn pads.

Ask yourself what has happened during your 31,000 miles of driving and you may figure this one out. Hit any bad potholes,curbs or something in the road?? Alignment

There is one other possibility. You have a tire on the front that is either coming apart or out of round(Oval). Rotate your tires one at a time.Put the right front onto the right rear and drive down the road.No vibration?? Right front needed replacing. Still vibrating? Rotate left front to left rear. No vibration?? Left front needed replacing.

YOU HAVE A WARPED FRONT BRAKE ROTOR , IF YOU FEEL IT IN THE PEDAL AT THE TOP WHEN JUST APPLYING AND YOU MIGHT FEEL IT IN THE STEERING WHEEL AS WELL ITS A ROTOR HAVE IT CHECKED BY A BRAKE SPECIALIST, FOR PROPER DIAGNOSES WITH A FREE ESTIMATE , THAT IF THEY DO THE WORK THEY MUST GIVE YOU A WRITTEN ESTIMATE THAT WHAT WORK THEY DO WILL CORRECT THE PROBLEM, SO THERE IS NO SURPRISES IN HIDDEN COST AND THEY MUST CALL YOU FOR COST OVER RUN OK, NOW IF YOU FEEL THIS THE SEAT AS WELL WHEN YOU BRAKE ITS THE REAR OK, NOW BY LAW ALL BRAKE ROTORS HAVE A MINIMUM THICKNESS AND THE CANNOT RESURFACE THE BRAKE ROTOR BEYOND THIS SET OF MEASURE, ITS THE LAW THEN THEY WILL NEED TO REPLACE THE ROTOR (S) WITH NEW DISC LINING OK. IF UN SURE TAKE TO THE DEALER OR A GOOD ASE SHOP THAT SPECIALIZE IN BRAKE BECAUSE YOUR LIFE DEPENDS ON THEM BEING CORRECT, TAKE CARE

I would start with a little quitchercryin. Its 8 years old and has 90K. Everything breaks down, and has room for improvement, if that wasn't the case, we'd all be driving model T Fords. 4wd isn't a necesity and if you don't have it you won't be in any danger, you may get stuck in the mud somewhere though. My 84 International truck just popped a brake line and the floor rusted out Also, the tranny went in my 87 chevy truck, the turbo in my 89 volvo is starting to leak oil, my parents 04 Audi has had Ignition problems, My bosses 05 kennworth has had to have kingpins and the 5th wheel rebuilt, brakes and AC work. And you know what, of all things, my 1958 John Deere has had to be completely rebuilt over the last few years. All these companies should be out of business for building this crap huh? Thats what you sound like to us mechanics.

Warped front rotors. That will be Rotors and Pads too. You can do the job if you have some tools. If not, get some estimates, shop around.

Does the break peddle vibrate too?? It could be warped rotors or bad suspension. Most likely it is the rotors You can have them re surfaced or if they are too thin, they will have to be replaced

both and you probably need a tire rotation and balancing. Start with that, its cheaper.