Bent axles?

[BCompy]

Ok boys your help is needed once again. I have a vibration in my car that has been narrowed down to bent axles. Tires have been balanced ,front end aligned, tires have also been removed and replaced with another complete set and what feels like a high speed shimmy continues. My Cobra specialist feels the rear axles are bent.( I am prone to performing the rare donut if there is absolute safety and a camera is present, juvenile I know , but I was sheltered as a child and not allowed on rollercoasters.)
Any way I digress. Ford no longer makes axles for the 79 rear end. I would need to replace the carrier and other odds and ends and that seems cost prohibitive. Does anyone know what other year Mustang differentials fit under the Classic. I am think of swinging a new third member rather than rebuilding what I have.
All advice is greatly appreciated. I have had two different front end shops go over the car and they can't find anything wrong. Ball joints , rack, tie rods, all tight, no broken welds, yet this shimmy comes on at about 65mph and planes out smooth at seventy . Can't feel it in the stick and vibration occurs if the clutch is in or out ,or in neutral.