Last year I bought an 87 syncro Westy.  It had a vibration at highway
speed, but I got it at a good price and thought I could diagnose the
problem eventually.  Well, I was right-I was, and it only took a little
over a year.  

At first, I thought I could just get the driveshaft rebuilt.  The vibration
would disappear completely with the driveshaft out.  With Budd Premack's
help I got the driveshaft rebuilt at Proven Force, which has received many
accolades on the list.  New U-joints, balanced, etc.  Vibration, however,
was just as bad when I reinstalled the shaft in the van.  I decided the
problem  must be some other component of the 4wd system.  Since I had
bought a couple of junk syncro vans, I had a plentiful parts supply, which
turned out to be fortunate.  I swapped in another VC/front diff and a
better set of front axles and CV's.  Improvement now, but still quite a bit
of vibration.  Finally I tried swapping in a driveshaft from another van
and the vibration magically disappeared.  I don't have a really good
explanation yet for what is going on here, but clearly one can replace the
u-joints on a syncro driveshaft and balance it in a shop and still not have
it perform well in the vehicle.  I'd be interested in hearing wisdom from
the list on what is going on here.  

Bill
St. Louis, MO 
www.geocities.com/yosemite/gorge/1706