Steven,

With a Syncro, you always need to be aware of the u-joints in the main
driveshaft.  They can be easily checked by putting the van on a hoist and
seeing if there is any looseness.

As a check, remove the driveshaft (by taking out 4 bolts in front and 4 in
the rear) and try a test drive.  If the symptoms disappear, then you have
the culprit.

If the driveshaft u-joints turn out to be bad, do not go to VW for a
rebuilt unit ( $ 700 ), but contact me re info on obtaining replacement
u-joints for a total cost of about $ 150.

I spent last summer resolving this matter with my Syncro,  so I have hard
earned experience with this issue.

Budd Premack
87 Wolfsburg, 84 Vanagon,  86 Syncro
73 Super, 64 Bug
Minneapolis, MN