Transmission failure, 97 Ford Explorer

Your radiator cools not only your engine coolant, it cools the transmission fluid also. If your radiator goes bad internally, it can get radiator fluid in your transmission, and transmission fluid in your coolant. You are not only going to have to get the trans repaired, you will need to replace your radiator, and flush the engine to replace all of the fluid.

Maybe they had a radiator problem and had the radiator replaced. Mean while coolant had gotten into the tranny and they didnt know it. It took some time for the damage to be done.
In this case, the radiator you have may not be bad. I would disconnect the tranny fluid lines from the radiator and run the engine up to operating temperature. If no coolant leaks out of the open tranny line holes, then it may be ok. Most shops have the ability to pressurize your radiator system to check for leaks. This will accomplish the same test.