[chowdah]

We call that thing a "guibo" (good search term). It is #2 here, and can be easily inspected under the car. Give it a peek.
http://www.realoem.com/bmw/showparts.do?model=DE93&mospid=47592&btnr=26_0188&hg=26&fg=10

 

[chowdah]

I'm saying that what you call a "u joint" is better known as a "guibo" or "flex disc" here (to aid your searches). I'm also saying it's an easy inspection to quickly rule in/out as a problem. Worn guibos are not uncommon.