i have an 86 iroc 305 fuel injected there is a vibration in the back of the car that starts at around 65 70 mph and i cant figure out what it is, is there anyone who can point me in the right direction and also the steering wheel shimmys at about the same speed but they are two different ones i have put the entire car in the air and everything looks ok to me

ckeck your tie rods. and have your tires balance

i put new tires on it and got them balanced and the vibrations went away thank you for your help