I read in the 2014 eco diesel owners manual you should inspect the front and rear axle fluid every 2 years or 20k miles. It says to change it at that frequency if used off-road, towing, police use, etc.
I changed my front and rear diff fluid on my 2012 3.6L wk2 around 50k after buying it at 30k. I do 5 to 6 weekends of light 4x4 and towing a small trailer every year. The rear diff fluid was definitely a bit dirty, a kind of grey/brown if I remember but the front fluid looked brand new.
No idea if the previous owner had the fluid changed. I have not checked but I'm pretty sure the differentials are different between 3.6L and 3.0L diesel so keep that in mind. My 2014 eco diesel only had 5k on it so not sure about that guy yet but I'll be sticking my finger in the diff to checks in 15k for sure!
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