used mobil 1 full syn in my diffs and was wondering when to change it.Daily driver,very little off roading just mostly city/highway.
30k? 60k? more?

Over 60k shouldn't be a problem. I'd change it every two or three years just for peace of mind to be honest. Stick your finger in and check the condition

Thank you. Have about 14k on them right now so looks like I have a ways to go

No problem. The oil itself should last a long time, but like most modern engine oils it's really a matter of contamination.