I have a 318iS with 90400 miles. It has a vibration problem that you feel in your seat at speeds of 38-45 MPH. Wheel balance was okay so I suspected that the problem was with the driveshaft center bearing. However, when I got the drive shaft out, it looks like the problem is the universal joints. The Haynes manual says that the U-joints are not replaceable and that you must replace the complete drive shaft section. Can this be true? This seems like a costly way to fix U-joints.
That's the whole point. BMW wants you to buy a new driveshaft not just a universal joint. Some machine shops can rebuild the driveshaft for you but better make sure they can balance it too. There are a couple of good sources to buy rebuilt driveshafts reasonably.