The vibration damper is on the rear of the Syncro driveshaft.

I hope that the NEAPCO u-joints turn out to be correct.  Many of us
Syncronauts have had a tough time obtaining the correct size u-joint.
Please keep us posted re your experience once you mount your driveshaft and
drive with it for a while.

Budd Premack
86 Syncro, 73 Super
Minneapolis, MN (Land of Sky-Blue Waters)