I have been chasing this problem for awile now.
customer had a vibration on accel and de-accel. he went to another shop and had both front axles replaced. (aftermarket) No change
then they replaced the front control arms ( volvo parts)
I then got the car. I replaced the front lower ( passenger side) motor mount. it was starting to go. no change
I removed the rear driveshaft and the vibration got a lot worse ( more torque on the front axles)
I then ordered 2 more aftermarket front axles from my supplier. It got a lot better afterwards, but still a minor vibration only on the accel.
over the next two weeks, the vibration got worse and now on the accel and de-accel ( not as bad as it originally was but still bad enough)
so i ordered two more axles with no change.
one option i know would be to put new axles from volvo but they are like 6-700 vs 200 for aftermarket.
any other ideas?

 

Is the vibration wheel speed or engine speed? If it's wheel speed I can just about guarantee it needs Volvo axles. If it's AWD check the angle gear, it's possible your vibration is coming from there. You can help narrow this down by pulling the fuse to the DEM, if the vibration disappears then clearly you know it's in the driveshaft or rear end.