Let me first start with a little background.

I have a 1988 740 turbo wagon with the M46. I had a similar issue with some really bad vibration in the back when starting from a stop. It was lowered. I put the stock springs back in. I then took the driveshaft [marked] and the new center support and ujoints to a machine shop where they replaced them all. I went home, and reinstalled this. The problem went away, kind of. When taking off, it almost feels as though the back wheels are rubbing something, but this is not the case. This car also has newer engine and trans mounts.

Now, onto the major problem.

I have a 1991 740 turbo wagon as well. Manual trans as well.

I replaced the guibo/flex disc when the other one was torn up. That was a few weeks back. Lately, the same problem as above has come up.

From a dead stop, it feels like a violent vibration is present, it shakes the entire car. It is not as bad when i am up to speed. When i get on the throttle while driving, it is present. Lay off the throttle, it is not as bad.
I just got this vehicle in early may, and have been trying to fix it ever since. I may end up putting in a new center support/bearing assembly and ujoints anyway.

I am kind of lost here.
I have seen the "shim the driveshaft" comment multiple times. Does Volvo sell the shims or do i just shim the center support with some drileld steel plate? Seems to be my last option.

Any suggestions? Could it be the torque rod assembly on each car?