My 2004 9-5 ARC idles rough (car shakes) when in gear.

It can be fixed, but it's impossible even to guess as to what could be wrong without experiencing the problem first hand and being able to do diagnostic tests. It could be an ignition misfire or vacuum leak. or any of a hundred different things. Is the Check Engine light on if so codes will be stored int the computer of your car which may narrow down the cause of the problem.

For the 4cyl Saab that's a "feature". It vibrates at idle when the car is in gear. Are you saying it's gotten worse than normal? Is it smooth when in neutral? If it's gotten worse than before, it could be a broken engine mount, or it could be your balance shaft has slipped position.