I had a similar viberation that I couldn't find. Being that the newly aquired '70 Chevelle wagon was for the wife's daily driver I decided to replace the entire front end..new bushings, tie rods, idler arm, everything. Then I replaced the front and rear wheel bearings. Still viberated
. I went out bought new tires and rims..balanced of course. Then had the drive shaft balanced..yep, still viberated. I gave up and said I'll have to live with it.
Last week I decided to put a new crate engine in and when I went to unbolt the motor mounts..they were finger tight!
I just finished up with the engine install, drove it on the highway and guess what?...No more viberation. Could have been the engine but I think it was the motor mounts not being tight.
Just my .02 cents.