My grandma’s car (96 Sable) is been well maintained, but we are debating if we should take it on a 300 mile trip. The only thing that concerns us is the fact that it shakes, pretty hard when we slow down after having been going over 45-55mph. Is this cause for an alignment, brakes, or just a tire rotation? Any help would be appreciated.
If the shaking is taking place only when braking, I would suggest that the brake rotors probably need to be replaced–or at least machined. There are other possibilites, but the brake rotors are the most likely suspects, so I would start there.
Also, a thorough inspection of the front end components by a competent mechanic would be a good idea. DO NOT go to Midas, Meineke, Monro, Sears, or any other chain operation unless you want to risk poor quality/overpriced/unnecessary repairs.
Vibration on braking usually indicates warpage or hard spots (or perhaps just rust spots) on the rotors. If the whole car shakes, that suggests that there is a suspension joint somewhere with some play in it that is allowing a major shake to get going in the car.
Even if the shake gets better with more driving (indicating that it was rust on the rotors), you should have a shop or knowledgeable friend raise it up and check the suspension for worn components before taking it out on the highway.