REAR BRAKE SLIP AT STANDSTILL- ON ICE

In the icy snow we get here in Portland, OR, my wife's Infiniti G37x awd sedan's rear end will slowly slide out on ice covered roads while stopped at lights. From the outside, I can see the front wheels are still, but the rears are slowly churning. HARD brake pedal pressure is required to keep the rear end in alignment with the front. Shifting into Neutral, I discovered, eliminates the problem. (On a crowned road, the car's rear end will slowly swing out so wide, I was afraid if I didn't put her in Neutral, the back end would end up in the ditch!)

Is there a problem with the brake balance? The dealer claims never to have heard of this. Recommends "bringing it in during the next ice storm." Which I'm not likely to do, because of the unsafe driving conditions and our proximity to dealer.