The angles for the prop and drive shaft are already kaput with so much movement.
If the front bushing bolts are loose then tighten them up .
If they are tight then you will have to replace the bushings.
There should be no movement on the front or rear diff bushings as you have described.
I am surprised that you are not getting a clunk when accelerating or de accelerating.
I would not rule out that the bushing issue may be contributing to your hum, but I have not read your other thread.
Alex.
The R35 GT-R runs upgraded fluid from the factory, however I have found that Motul RBF 660 is superior.
The GT-R oem lines get changed at every 60,000 miles or 5 years at the G2 service.
You do not require the fan deck removed.
The belt tension is adjusted by the movement of the spring loaded tensioner pulley.
Simply check the position of the notches to determine if there is a belt issue.
Alex.