Steering Wheel Shimmy for 2 years

I have had this problem for about 2 years now. This occurs during driving and not braking. As soon as I get to about 70mph my steering wheel shakes and my seat starts vibrating. I have had several new tires put on and balanced / aligned and it doesn't help. I have had new brakes and rotors installed and that didn't help. The rims are not bent and are stock. I have checked the tire pressure and that is good. The suspension has also been checked. I just can't get rid of this shimmy / vibration and really don't know where to turn next. Could the struts be good enough to align but starting to go bad that they cause a shimmy? Maybe steering linkage or replace the hubs? How can I check these 2 things to see if they are bad? I have a 99 Grand Am GT with 80K.

Any suggestions would be greatly appreciated.

Thanks.