Mazda Vibration in Gear

I posted this on mazda626.net.

I have a 97 626 Automatic 4 cyl. Why does the car vibrate when I'm stopped at a stop light and the transmission is in "Drive"? The vibrations goes away when I put the transmission in neutral or park. I'm trying to figure out whether or not the problem is due to cracked engine mounts. I see a crack on the passenger side top mount, but I can't see whether there's any cracks in the rear firewall mount and driver side top mount (near the battery).

I have read some posts from people with similar problems and suggested solutions.

Some posts say the problem is bad idle. My idle speed in park or neutral isn't noticeably less than the idle speed in drive. I revved the engine up to 1000 rpm in park/neutral and the car did not exhibit more vibrations. So I don't think it's idle speed related.

I've also connected a vacuum gauge to the PCV valve vacuum hose and didn't see anything abnormal. The needle was steady. Perhaps I should repeat the vacuum test and put the car in drive with the brakes applied?

So I suspect the engine mounts are the culprits, but the rear one and driver side top one is hard to inspect and to take out. If I just replace the top passenger side one, I'm not sure if that will help or not.