unless you are using a vernier torque meter, it is hard to get a diff exactly the same each time, even using new parts each time, plus the diff has to settle too, which adds to the work, whereas a geared diff, once built, will operate right away, and if rebuilt, with the same oil, will operate exactly the same way.
The only down side of the geared diff, is it removes any 'give' in transmission if you are not running a slipper, hence why the rallyx stuff is well over engineered.
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