I suspect that what you are getting is massive wheelspin because the diff is actually working.
Torque reaction will be at 90-degrees to the line of thrust and if the diff is rooted (Oz-speak for worn-out) and only one wheel is spinning, then the non-spinning wheel will tend to keep you straight.
If both wheels cut loose and all the tyres are doing is spinning, the torque reaction will tend to push the rear of the car sideways.
Sounds like the car is grip-limited at the rear on a hard launch and the fix is to provide more grip - better rear tyres will help, as will a less aggressive launch.
I have a MY09 OZ-spec Z34 with a Quaife diff - currently fitted with Nitto NT05s for daily use and NT01 fr track-work and with traction control off, hard launches will fry the rear tyres, so throttle moderation gets me off the line quickest (circuit work rather than 1/4 mile). I also have to make sure that the tyres are warmed up on the out-lap before leaning on the car - if I don;t do that, then it is instant oversteer.
If you launch hard, are you leaving rubber on the road from both sides of the car - if so, the diffis working as designed and I am maybe right - if only one wheel is spinning, then I am wrong and it is a different problem ...
A car that steps out on a hard launch is definitely grip limited though ....
RB
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