I have a 2000 ZD30 auto. A number of people I know have a similar model and they all have had problems with the auto cooler in the radiator failing and allowing water to enter the auto box. Some caught it early and didn't do damage but one resulted in a total rebuild - at huge expense. I decided to renew mine as a matter of caution and as I tow a large off road caravan I decided to go for a heavy duty aluminium radiator. Works great so far.

How old is your radiator? You can have it tested but to do it properly they would have to remove the tanks to see if there is any errosion (especially in the cooling tubes for an auto).