Garage service feed

You mention service, is this a service or feeder?
It’s likely a feeder and since 2005 NEC the r requirement has been for as 4 wire feeder, seperate EGC and neutral, s as bc a grounding electrode system, ground rods may not be needed.
 
If it has a panel, you need a ground electrode system. Feeder -vs- Service determines whether it is 3 wire or 4 wire supply.
 
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