Transfer Case problems of the 2006 Ford Escape Hybrid

My husband and I when returning home from a visit to relatives in taylorsville, utah. It had been snowing but had stopped or slowed down in taylorsville. Since we were driving an Escape Hybrid 4-wd, we believed we would be fine to begin our trip. We were driving on interstate 15 when it started snowing really hard. We were just past the provo area. There were many cars and big trucks on the freeway and everyone was driving as slow as possible. We were driving in the slow lane when suddenly the car started to slide into the inside lane. We had not made any sudden moves, had not increased speed or applied brakes. I could not understand why we began to slide as if the 4-wd kicked in , as it should had. We should have had the necessary traction. It did not feel as though we had any traction at all. As we slid to the inside lane we continued sliding on an angle toward the shoulder. When we reached the shoulder the car rolled onto the passenger side. I was amazed as both myself and my husband grew up in snow country and had many years of experience in driving in the snow. I never had a vehicle suddenly start sliding unless I was trying to turn a corner, stop at a light/stop sign, or made some other action which changed the movement of the car. I have never rolled in a car, but I have slid onto the shoulder on a few occasions. I did not believe the Smart 4- wd system was working properly, and I did not believe we should have rolled.