You may want to think about changing your itinerary. State Shuttle (at Ground Transportation/Welcome Center) has service between JFK and EWR for about $30 however the trip takes at least 1 to 1.5 hours and you will need to clear customs/immigration on arrival at JFK which can take 45 minutes to 1 hour. Given these time frames I don't think you will get to EWR in time for a 1530 departure.
You may want to think about