Ok shanhsv i hope this helps.
A hydratrak has a 7.75inch crownwheel. Being a viscous coupling it is not subject to the same loads as a normal lsd.
It was introduced in the vr range, with cars with the 215 motor.The auto models ran 3.08 and the manuals cop 3.45.
The vr/vs has the 76 model hydratrak and the vt had the 80 model ( different size crownwheel). There is also a M90 model through Spicer Axle Australia.
A viscous style lsd coupling is the basis for the hydratrak centre. The left axle has a smaller extended splined section at the end of the axle that goes right through the centre into the hydratrak cartridge on the right side of the diff centre yhat contains the viscous coupling which creates the lsd action. On the other side, the right hand axle only has theone splined section and it goes into the hydratrak cartridge as well, but in this case the splined section of the cartridge is part of the outer section. this allows the wheels to still turn inderpendent of each other but, being a 'speed sensitive' centre, it progressively feeds power the wheel with less traction as wheel speed increases. the best thing about the diff is that there are no friction parts..
They are strong and can handle good power, but are expensive.
Motorspecs are the suppliers for retrofit kits and they have access to all three models od hydratrak diffs. Hope that helps mate. I have seen a guy selling one in the trader for around 2000. So not cheap.