Mystery vibration at 25 mph can't be diagnosed. It's not the front end!

Firstly, I need to thank slantsixdan for the information provided. Now I can use it! The mechanics fought me the whole way. Done with that. Nope, I haven't bought the $449, 5 leaf springs yet. I just said that I found them right away because the rake has always bothered me. When I get there I'll put out feelers first on the forum and go from there. I'm not there yet! I want to have a smooth ride first. That is priority for the winter. Then I'll move on to the springs. Or I might just do them concurrently. I'm not blowing money yet on the parts. Yes the E3 spark plugs was a bad idea, but I have a neighbor that likes my car and he suggested them. No harm, no foul. I immediately told the shop to put the stock champion plugs back in.