Go for easy first
If the lower shock mount is sloppy it can present as play at 12/6 absent at 9/3.
Press your thumb on the bottom of the shock body and across the mounting bracket on the swing arm.
Now give the final drive a waggle up and down.
Do you detact any movemnt between the shock body and the mounting bracket
With luck that will be all it is.
If the slop is present get rid of it and check for final drive play again
Hopefully it will be gone.
Next test give the wheel a spin forwards then backwards.
Is there a difference in the rumbling noises?
Big difference has always meant bearings for me.
With an even noise in both directions I usually get by with reshimming the big bearing but have alwways had the 12/6 & 9/3 play together in this case.
My bike is slow but the earth is patient.