Originally Posted by duxthe1

That describes a worn driveshaft center support to a "tee". The worn support allows the driveshaft to turn in a orbit until it eventually gets enough speed where centrifugal force pulls it back to center, usually occurring and diminishing around the speeds you have listed.
Try having someone in the back seat to see if they can feel the vibration in the driveshaft tunnel area or have somebody else drive while you check.