Does the car have any kind of whine or noise to her when driving or accellerating? You could have a rear differential/ bearing problem. I had a similar issue with a 1990 Volvo 780....

-Just a thought....

P.S. After driving for a while, do you smell anything burning? That would be a sign of a binded parking brake. Your cable may seem ok, but the brake shoes might not be releasing from behind the rotor. Best thing to do at that point is nock one of your rear rotors off and investigate. By no means does it sound mechanical.

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