Front diff isn't too hard to change (if you need to eventually replace it), but does take some work. Lots of wrench turning, but if you are a decent backyard mechanic, you can do it. Weird thing on our cars is that the front axles go THROUGH the oil pan. Odd. Diff attaches to oil pan. You don't need to remove oil pan, though.
If you need a front diff, there is this car on ebay (listing is for transfer case, which is pretty cheap BTW) and he may have front diff. Just ask him. http://cgi.ebay.com/ebaymotors/ws/eB...K%3AMEWAX%3AIT
The lsiting says it's for an 06 325, but the car is a 07 328xi with 33k miles (see VIN tag in listing pics).