'03 Gt rear end vibration

[ptklaw]

I own a 2003 GT with automatic transmission. Since new I have had a vibration from the rear end that extends up to the driverseat only when driving over certain streches of roadway. Generally the vibration will happen over the same road at the same spot and lasts about 2-3 seconds. Is this a common effect of the solid rear end or is it a defect that can be corrected?

[SpectorV]

does it happen at a given speed or just random?

I would look under the car for wear marks the driveshaft might be touching on the ebrake bracket or something like that a little bit from time to time causing the noise~