Sounds like you may have a faulty vacuum pump or brake booster. Not unusual on BMWs. Before buying other tires I would determine if the brakes are working properly.
Doesn't matter if it is 4X4, has ABS or any other stopping or starting assists. If the tires are not effective on ice or snow, especially on a vehicle as heavy as an X5 it is not going to be fun. I suggest reading through the following thread and perhaps make a new post:
http://www.xoutpost.com/bmw-sav-foru...endations.html