Dear Eric,
thanks for extramly good and quick answer.
I'll look have to look over your advise later today.
The ZF is very quiet and I'would assume that the general condition of it is OK.
But I'll have to find out of this fault. I'm also planing on changeing the fluid and filter just to be shure that it is OK.
The vibration is arround the drive shaft. I just bought the car and it had a new center support on the dirveshaft (work made on an special BMW workshop) so I assume that the joints are OK.