You do not *have* to remove the cab for any service, however it is often easier to do the service with the cab off. That said, the problem is more likely to be cavitation damage to the water pump back plate than an issue with the oil cooler.
This cavitation damage is fairly common, especially if you have not been properly testing coolant nitrite levels and adding VC-8 additive if they are getting low. It also appears that the water pump back plate has been redesigned to reduce cavitation issues, the new design has a "tab" in the flow path in the bottom of the impeller area.
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