Although for most of the people here the problem was caused by the front driveshaft in my case it was the rear. I think that's just $150 or so for that piece.
I had the same problem and I replaced the front first with long splines shaft (no luck), then the chain in TC (no difference again) and then the rear driveshaft joint. I think it's called constant velocity joint - 26117526822. I replaced the rubber donut joints as well but the old ones were as good.
So, just to get rid of the noise from D to R I would do the 26117526822 first. It's the cheapest, then the front drive shaft splines if the noise doesn't go away and then the chain in the TC... but that's based on my experience.

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4.8is 2004 imola red
4.8is 2005 Black on Black - DINAN/2 sold
4.4 2004 X5
2003 330xi
2008 328xi - sold
4.4 2000 X5 - sold