I use a Wavetrac type differential:
WavetracŪ Differential - A torque biasing differential with a difference
This is basically a Quaife type ATB differential with Salisbury type thrust ramps built in to allow the differential to work to some extent when the inside wheel is unloaded.
With a Quaife or Guard ATB differential, as the inside tire load drops down below 20%, the differential looses effectiveness and starts to act like an open diff. The Wavetrac tries to compensate for this.
There is a video on the internet somewhere of a guy with one of these on a BMW, where he jacks up one of the rear wheels and is able to drive off in a fashion with one wheel up in the air.
I know from photos of my car on both race tracks and auto-x, the inside rear is indeed very lightly loaded at maximum G's on r-comps, and I have my suspension tuned to minimize rear weight transfer.
So it is a real concern if you are pushing the limit.