you figured out the shaking source??
If not I would suggest checking the driveshaft bearing/carrier (someone correct the name of the part if wrong), anyways, it will be located at the middle of the driveshaft, with the driveshaft going through it. when these go bad, the car vibrates increasingly as you go up in speed, and specially if you apply more power.
it will happen very strong, if you take off hard.
Let us know.
And please post some pics, would love to see another 810.
NOTE: if nothing makes sense feel free to say so, English is not my mother tongue.
Best,
Moe