Weird Vibration (LONG)

[skokie]

Ok this is not on my 68 truck but on my 97 thunderbird. I took it into the goodyear shop that I got the tires from because of a vibration that I was feeling that started at 50 (after 50 the faster you go the harder it shakes) and would act like this. You would feel a slight vibration but it works its way up to a shaking the steering wheel vibration but you can tell that is not where it is coming from. The wheel is shaking as a result. Its not coming from the front of the car. So I ask if i threw a weight or two. So for good measure they strip all the weights off and rebalance it all. I drive it off and same thing. I come back and tell them this so now they take the tires off of the rims and check for a warped rim. Everything is fine there to. So they remount the tires and rebalance same thing. According to them the tires are good. No separation , runout etc and they are balanced. Suggest it maybe a drive train issue or maybe a torque converter. So I call a tranny place that I know and tell them about it. My friend says bring it down I will check it out. He loves a good challenge He checked it out and cant find anything drivetrain wise wrong. U joints are good, drive shaft is within spec, no tranny issues etc. aka looks good. So he takes it down to another tire place for the heck of it and they check the tires. They found 2 out of balance and they correct. Well I go pick it up after they closed and same issue. So now Im almost at the end of my rope here. What the heck could be causing this vibration? Any body seen anything like this before.

Thanks