Brake Grinding Noise problems of the 2005 Nissan Xterra

I have a 2005 Nissan Xterra 4-wheel drive. While driving it in snow, ice, wet roads or simply stopping quickly, I hear a horrible grinding noise from the front end. It happens when I apply the brakes coming to a stop. It has to be a condition where the wheels may lock up. Also, the brakes let the vehicle "loose" for several feet and/or seconds before the grinding begins. I am used to the fluctuation of the brake pedal with an abs system. However, I had never heard a sound that reminded me of grinding aluminum cans in a coffee mill. I took it to the service dept when this first happened (December 2005; last incident, April 2010) and they have assured me that this is how the abs is supposed to sound, feel and work. My husband noticed the same feeling/sensation and asked what was wrong with the brakes. I told him Nissan says that is how they're supposed to work. It doesn't sound acceptable that an automobile company would produce a safety system that sounds/feels like your differential is being ground into little metal pieces.