If it started after a shock like hitting a pothole, I don't think changing the fluid will help. Some shops have a stethoscope-type machine that attaches at several points on the rear end that allows you to listen to different sections at a time and can help you pinpoint the sounds location. If nothing if visually out of place, that may be a good start.
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2012 Kona Blue 3.7L V6:
Borla LT headers, O/R h-pipe, Borla S-types, 3.73 gears,
Steeda adjustable UCA w/ mount, BMR LCAs, Airaid CAI,
LCA Relocation brackets, AE driveshaft, GT 80mm TB,
Black 18x9/10 2010 GT500 Wheels w/ Nitto NT555s,
Eibach Sportline Springs/shocks & Koni STR.T Struts