Well with the four link you should be able to change the pinion angle pretty easily. I would give it a try in both directions and see which ones tunes out the problem.
With four links i like to use a Fabric tape measure the kind used for sewing just to double check things quickly. Measure all the control arms and alingments and make sure its in the car straight and adjusted straight with the front wheels.
If no love from pinion angle adjustments i would take the car and have it put on a four way alignment rack and make sure its square and properly aligned. Also check the front tires you never know.
If all that fails try a euro shop and see if they have a on car wheel balancer and have a custom drive shaft built. A good drive shaft shop may also be able to find the problem just by throwing the shaft on a balancer. Fabbed up drive should not be that expensive.