wrong diff oil will not automatically cause whining, it will cause increased wear of components, which in cause can lead to whining.
the back off noise at cruise is a classic sign of excess backlash, which means there is too much freeplay between crown and pinion gear. that can be adjusted through shims in the diff itself.
continous whining under accel and decel can be caused by diff carrier bearings or wheel bearings in live axels.
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N12 Pulsar - sold
Gen1 Liberty Turbo - sold
VP Commodore Turbo - sold
LN65 Hilux Turbo - sold
EL31 Corolla Turbo - sold
Ford AU Ute Turbo - sold
Ford AU XR8 Sedan - 5.4l V8 Turbo (in the build)
Ford BA XR6T Ute - daily driver
Ford FG XR6T Sedan - cruiser
do you see a general trend? I DO 
Can't live with it, can't live without it!
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