I have a 2001 keep grand Cherokee that won't drive in reverse but drives forward what does that mean

Take that one out and put a good one in. add a cooler. I saw one do this during its twenty years twice. usually following overheat from radiator and waterpump failure. The passages were internally plugged because the atf could not flow from being cooked and strangely solidified. Other service bulletins and recalls applied to that one and were not performed. after years, it lost reverse first. hope yours is not that bad.