I didn't realize you were talking about a '90 300SE, I was going by your handle. Guess I should read more closely.

The 300SE is VERY prone to having vibration issues when the motor mounts are shot. Also, the exhaust likes to sit on the idler arm when this happens.

After I changed out the mounts on my 300SE, it felt like I had a new motor.

If your motor mounts are shot, do them along with the trans and diff mount and your issue will likely go away.

-tp