| i think theres a good chance its your wheels. plastic hub centering rings will never make the wheel fit as good as a hub centric wheel. i think it's worth trying to fit new ones either a size smaller, a size bigger, or just the same size if your current ones are starting to wear away.
i had aftermarket wheels for a short period and sold them because i couldnt get rid of the 50mph vibration (and a couple other reasons). i, like you, had the tires balanced multiple times, had 2 or 3 allignments done, new suspension, and still couldnt make the car feel right. When i eventually sold the wheels the new owner said he bought some new hub centering rings for them and the vibration had all but dissappeared. I didnt really care at that point because i had no plans of keeping them with or without the vibration, but I would deffinately try buying some new 57.1mm hub centering rings before you start messing with more major things like tie rods and strut mounts | 
