I have a 1986 IROCZ with R700 transmission. When I rebuilt the whole front end (new center link, tie rod ends, all bushings, struts, gearbox, pitman arm, ect).

The vibration does not come from the front end, I feel it coming from the rear end.

What should I check first? Bad Pan hard bar? bad rear axle? worn drum brakes?

By the way I have checked the wheel balance twice already

 

Did you check the u joints on the drive shaft?

 

Yes I did, I even replaced the u joint about a month ago.

wondering could a bad Brake Drum cause a vibration? Since brake drums do come with counter weights welded on them.

 

Did you make any changes to the vehicle ride height, rear suspension, or torque arm mounting position? It sounds a lot like a problem with the driveline angle.

- Kevin

 

Another thing to check believe it or not is your harmonic balancer on the front of the motor. That can give the same effect as a worn u-joint. Do the U-joints first though, they are not that expensive so its a cheap insurance thing to rule out.

 

I had a bent left rear axle flange.

 

Check your tires for roundness, flat spotting or bad belts can cause issues, though they would occur at all speeds. The range you described would lead me to guess the driveshaft. Steel shafts are notorious for vibrating in the 60+MPH range, and factory aluminum ones are balanced to a much higher RPM. It might have something to do with pinion angle on lowered cars or those with sagging springs, I'm not entirely sure as I didn't pay attention to whether or not those who reported issues had adjustable torque arms or what height their cars were at. I just know that a lot of people have found factory aluminum shafts to be a cheap fix. (If a factory one would fix it, so would an aftermarket one... but it would cost a lot lot lot more)