Car vibrates when in gear

I have a 92 Integra GS with a replaced motor. When in neutral and engine running, no vibration at all. But when I stick the gear into D, then the car vibrates and when I stop at a
stop light.
Any ideas why the car vibrates when in gear?
Motor mounts? If motor mount was the problem, wouldn't it vibrate all the time, even in Neutral. Rear transmission mount? Anyone know where this is located and is it easy to fix?
Thanx in advance for any advice.
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