"when idling".... is that to say it vibrates when you're just sitting parked with the engine running, or is that to say it'll vibrate as you coast down the road with your foot off the gas?

If that latter, I'd vote for rear U-joint. Mine was doing the same.. started a couple weeks ago, I checked then and all U-joints seemed tight. Checked again last week, and there was noticeable play in that joint... it was quite sloppy, quite bound up, and upon removal, very worn and rusty.

That's not to say you wouldn't need motor and transmission mounts too...
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-Matt I ♥ my ♂

My 740 developed a really bad vibe coming off the expressway one time..Had normal power, seemed to run ok...It was a section of missing cogs in a V belt that caused the a/c compressor to shake badly. If it seems to run ok without a missfire and has good power it couldn't hurt to check the belts (esp if theres oil on the alt)

Passenger side; you should actually see that the metal plate that is part of the mount slid down the Rubber part of the mount. Again on the passenger side, see if you can get your finger between the mount and the oil filter.
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.no.net/ebrox/Tony's%20cars.htm

A bad mount would not normally cause a vibration. A engine miss would, are you running on all 4?
Depending on the style of mount they are round and bulgy looking, an oil soak would not harm it or cause it to bulge in any event. Only in the long term oil will cause a problem. I would clean it off and look elsewhere for the vibration.
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David Hunter

You will need to inspect the motor and transmission mounts from under the car to look for damage. Other than bulging look for torn or broken rubber. I just replaced a failed transmission mount that was causing terrible vibrations.
Dan