Mysterious vibrations

I am experiencing vibrations similar to driving over a rumble strip. It starts occurring at around forty mph and subsides at around sixty five to seventy nine mph, re-occurring when letting off the gas.
It started "out of the blue" after a highway drive. The following has been checked/replaced so far: U-joints replaced, rear end been opened and checked for metal shavings and oil, starting up vehicle with front-side disconnected driveshaft, tires have been balanced, rear axle bearings and wheel bearings been checked, all without any result. No vibrations with disconnected driveshaft. Vibrations are still present. What to do and what could be checked next? I have already spent $600.00 without any result. Getting desperate.