It's interesting that several people are having this problem.
Both my 200k miles 318iss do it, especially when backing up a gentle incline. The black one has since I bought it 2 years ago. A very good mechanic couldn't find any problem. I wonder if the judder caused a new $26 flex disk to deteriorate so quickly (like 10k miles). It still does it with the new $46 flex disk, just not quite as bad.
The red car was perfectly smooth when bought last year, but ever since I got an engine rebuild and a new clutch a few months ago it does the same judder--worse in reverse. Just last weekend I noticed that it now has cracks in the flex disk and that one of the CV boots is torn and that CV joint is very loose.
The driveshaft U-joints and diff mounts on both cars look and feel good, but who knows for sure? One mechanic told me that it could be the flywheel, and that it should have been replaced with the clutch.
We have got to narrow this problem down--trial-and-error will obviously break anyone's budget. I'm not enough of a mechanic to keep taking my driveline apart, and am about ready to give up and buy a (low mileage) e46.