while driving between 45 and 60 mph the car feels like you're driving on rough pavement. no performence issues just that rough road feel in the seat of your pants. 6000 miles ago i installed a used drive shaft and used 3.25 diff. only started this feeling a week ago or so. under 45 and over 60 she is as smooth as silk. what ever it is it is not rpm related. depress the clutch and coast and the ride is still rough. any ideas??