It's just a thought, but are you POSITIVE that the sound is coming 
from the rear of the vehicle.

A couple of months ago, our van started making a similar noise that I 
was sure was coming from the right/rear.  I took apart the CV's 
(looked OK), must be a wheel bearing, pulled the housing, went to the 
machine shop for the press work, put it all back together.... 
reet-reet-reet!

This time  I had my wife drive the van, I rode shotgun with my head 
out the window... and the noise was coming from right under me!!  I 
pulled the wheel and found that the genuine VW brake pads I had 
installed less than a year ago (20,000 miles?  My odometer wasn't 
working at the time) had completely worn out and the rivets were 
rubbing on the outside edge of the rotor.  New pads and 15min later 
the noise was gone. (Yes I did feel stupid).

I'll assume this isn't the problem with your van-  I just thought I 
might save some unfortunate a needless and very costly repair.

Dan Fuller