I have a 1986 IROC-Z, I just rebuilt the front end and turns on a dime. But, when I take the car on the highway and go over 60, I feel a vibration.

I know it should not be the front end because I replaced almost everything (Struts, tie rods, gearbox, bushings, etc.).

Could it be bad rear shocks?

I have not touched the rear suspension.

 

i can only think off Drive shaft/ pinion angle or unbalanced wheels beeing the problem, if the power steering high pressure line is touching anything on the motor or chasis it will cause vibrations aswell