My car shakes pretty badly at around 65mph and up. It seems to be worse when decelerating. Could this be my driveshaft, or is there something else to look for? I was thinking of installing a 98+ driveshaft. Is this a direct bolt in? Thanks in advance for your help!

 

You might want to raise the car up and check the U-joints. Your probably on the right track but it might be something like u-joint which are a not-so-costly fix. How long has it been doing it? does it cluck when changing to reverse to drive ect.? A quick call to inland empire will net you a killer driveline for not all that much and it comes with the U-joints already in

 

could be something as simple as your tired out of balance too.

 

I had a odd shake around 55 that I chased for a while. Turned out to be a lose door skin. Drivers side door skin near the mirror had come lose for about 3 inches. But was just enough to grab air over 50. Never noticed it till one day I had my arm hung out the window. I noticed if I kinda squeezed the door it stopped. Dab of silicone fixed it.

 

When my u-joints went out it shook the whole car. If the vibrations feel like they are coming from under the console and behind/below the seats then it is likely your driveshaft. Thats how the vibrations felt in my car at highway speeds.