Go underneath the car and see how much clearance there is between the oil pan and the crossmember. There should be at least a half an inch. If they are touching, or even very close, you will need to replace the pedestals and brackets for the motor mounts. These are the four cast aluminum parts that connect the motor mounts to the crossmember and the engine block. I have an 88, that had a similar vibration. I tried everything to cure it, and only discovered the pan was on the crossmember while in the process of converting it from automatic to stick shift. I used the pedestals and brackets from a 90, 5 speed car, which raise the engine about 1/2" compared to the ones that came on my 88. Of course, as I was installing a stick, I changed over to solid motor mounts. The original liquid filled mounts did not appear to have anything wrong with them, so I suspect Volvo changed the design of the pedestals and brackets due to problems with the engine sitting too low.