Is it a Gran Tourismo MC Shift or autobox? Not quite sure what car you have..
Either way a diff clunk is very metallic and noticeable. Probably bushings as Zep states.
Prop joints, diff backlash, driveshaft, loose yoke, diff bushes, diff cradle too on a Ghibli. Try turning the prop shaft by had with the wheels locked and see if there’s any play in the drive itself, that’s easier to rule out than bushes.
Yes the Auto ZF is 3.9 (or 3.95 ?)
There are less knocks with the ZF diffs. But on auto cars people don't notice the knock because the torque converter hides it.
Well that could be many things, you need to go under the car and inspect the 3 shafts and the play inside the diff itself. A light play in the diff is normal.