My 2003 S40 has been shaking when I get over 40 mph. It smooths out again once I reach around 65. The shaking seems to only occur when accelerating, but kind of stops at higher revs. I was thinking it was an issue with a motor mount, however when a friend who is more knowledgeable with cars looked at it, he noticed the driver side strut mount looked damaged.

Could this be the sole culprit for my shaking issue? It makes sense to me that this could cause the car to shake, but I don't see why it would only shake when my foot is on the gas.

Also, the car seems to jerk when shifting from reverse to drive when the engine is cold.

 

If the mounts are original then they are probably overdue for replacement anyway. For me the car shook while in gear with my foot on the brake. If you get up under the car you can look at both of the lower mounts. Mine showed significant cracking in the rubber. I also replaced both upper mounts but found out the trans mount did not need replacing.

For the strut take a look at the strut tower where the shock sticks through the hole. It should be centered in the hole. If it's not centered then the rubber mount has worn away. Mine would also make a thumping noise over bumps that seemed to come from behind the dash.