Need 350 Trans advice.

I have a 77 camaro with a 350 trans in it, The car have a bad vibration in it starting at 70 mph. I have done a lot of things to get raid of it but nothing has worked.I have installed new ujoints,had driveshaft checked for balance,I have installed new frame mount brushens,changed rearends,driveshaft yoke,and torque converter.Is there any way the trans could be causing this problem?The trans has been rebuilt,but I can't recall if it was doing it before it was rebuilt.I have done everything but replaced the trans. need new ideals I have run out of them.I also had all tires checked and balanced,I had axles checked, and like I said I have since changed the rearend and the problem is still there.:think: