In flight vibration in the rudder pedals caused a B737-400 flight crew to return to the departure airport for maintenance.

Narrative

The First Officer (pilot flying) asked me to put my feet on the rudder pedals to feel an abnormal vibration. I felt a rapid frequency vibration through the rudder pedals...all engine/aircraft instruments appeared normal and no airframe vibration felt. Aircraft was in clean configuration; 250 KIAS climb through 5000 FT. As we accelerated after climbing through 10;000 FT; the vibration decreased to barely noticeable/nonexistent at 300 KIAS; which mirrored a previous write-up in the logbook. I contacted Maintenance Control and informed them of the vibration and normal engine/aircraft instrument readings and asked for their assessment. After several minutes; Dispatch informed us that we were to return to our departure airport and amended our release to indicate so. I asked if they wanted us to land overweight or burn down to max landing weight and was told to land overweight. After coordinating with ATC for our return; I informed the flight attendants and passengers of our abnormality and our return. We reviewed the overweight landing procedures/considerations prior to our arrival and landed uneventfully. After arrival at the gate; I entered the vibration and overweight landing discrepancies in the logbook and we debriefed Maintenance regarding the vibration. The First Officer stated the aircraft had required an unusual amount of right rudder to stay on centerline during takeoff and this was entered into the logbook as well. We then proceeded to our replacement aircraft to complete the flight.