On my 1998 Mercury Mountaineer the car is now shaking when I use the brakes.
You most likely need new front rotors. Have your brakes inspected.
Most likely your brake rotors are warped. Although it's sometimes possible to turn your existing rotors, they often warp again pretty quickly because they're now thinner, so it's usually best just to replace them from the start.
As others have said, it sounds like warped rotors. Have your mechanic check the brakes.
Is it ok to drive for a few days til I can get it looked at?
If you're careful - your braking distance is increased when you have this problem and it does abuse other parts in the front end.
Had the same problem on my Taurus, but with no pulsing in the brake pedal I knew it was'nt the rotors. Turned out the inner tie rod ends were shot.