2003 Subaru Impreza with the 2.5 engine. Crazy problem. What's the problem?

We came home from grocery shopping yesterday afternoon. We put the groceries away and then went to go to dinner with my inlaws and it shook like crazy as soon as we put it in reverse to back out. I looked under the hood while my husband put it in drive and the engine shook back and forth. I thought it was gonna fly... show more We came home from grocery shopping yesterday afternoon. We put the groceries away and then went to go to dinner with my inlaws and it shook like crazy as soon as we put it in reverse to back out. I looked under the hood while my husband put it in drive and the engine shook back and forth. I thought it was gonna fly out. This morning I started it and it drove fine, no problem at all but the Check Engine light is on.
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