My JK has 16,700 miles. I took in to my dealer today for TSB bulletins. In addition the service adviser advised me that Chrysler recommends front and rear differential service after 15,000 miles. They quoted me $338.00 to do this. Questions: Does the JK come from factory with synthetic differntial oil? If it does why change after only 15,000 miles? Last is $338.00 a fair and reasonable price for this service. Thank you in advance for your reply.