About a year ago, I replaced a head gasket on a 1994 Chevy Cavalier 2.2L. After I put everything back together there were two parts left over. The car runs good considering it's 20 years old, however I'm assuming these were bracing parts; when the car gets above about 55mph or so, it shakes.
It does the same when you rev the car in neutral, so it's nothing to do with the tires, and they were all replaced recently anyways.
The car shakes when it's cold, before it warms up, or if its been sitting awhile, so I suspect it's these two parts. It also loses speed quicker than before.
I'm not sure where it is that they go exactly. My assumption is on the passenger side within the engine bay.
Here are two pics of the parts.