My '93 940 also has had an engine vibration, annoying but not fatal. In my case, I noticed the vibration right after installing new engine mounts. I thought the old (original) mounts had allowed the engine to ride too low. I think think that the low priced mounts I installed were either solid rubber of just poor quality and are too firm. Our 240s all have much less vibration.

The advice to go with OE Volvo mounts makes sense. I've just got too much else to do so this has been a low priority for the last 30K miles.