W210 E320 Station Wagon 4matic ?????

[BigRed0803]

I am looking at a I believe 99' year model E320 4matic Sedan. The car has some sort of transmission issue. When you shift the car from park into reverse or drive, it sounds like the transmission is chewing ROCKS! Very loud and violent. The car won't pull itself out of its tracks either forward or backward. My question is what happens to a car, or how would you know, if the transfer case goes out on one of these 4 matics? I have read quite a bit on these cars and it seems like alot of them have had transfer cases replaced and I was just wondering why?