Try this: Lay a straight edge on top of both struts at once, near the bottom end where the rear strut attaches to the front strut, and look from the side at the trailing edges of the struts to see if the rear strut is slightly twisted (from tightening the jamb nut), then lay the straight edge on the bottom of the struts and look again. (This will help you see if there is a twist or not). If the strut has a twist to it, loosen the jamb nut, take the twist out, then retighten it, holding the strut from twisting as the nut is tightened. This is a common problem with streamlined flying/landing wires on biplanes, they vibrate if they aren't perfectly streamlined with the airflow. I imagine the same priciple could apply to streamlined lift struts.
At the Pitts factory we used a string on the slave struts between the ailerons to force them to 'fly' in one direction, thus cancelling out any vibration. The slave struts are a streamlined tube, and we added a piece of string (round waxed rib stitch cord) just aft of the 'center of lift' of the tube, (inboard side), and covered it with a color coordinated length of tape, almost the entire length of the slave strut. If taking the twist out of your rear strut doesn't solve your vibration problem then 'string' your rear strut to make it 'fly', either up or down. You could use a length of welding rod instead of string and tape that to the rear strut, just aft of the 'fattest' part.
Good luck, and let us know what works,
Blair
