The Most Common Metal Line Fitted to the Radiator with a double flare fitting is an Automatic Transmission Cooling Line. I can't think of another purpose, other than Engine Oil Coolers, which are usually flexible lines. It would be extremely extremely rare for this kind of line to be anything else. I'm a Noob to XJ's, but have lots of experience on other cars, so take my advice with a grain of salt.
The fact that your diagram shows it running to an auxilary external trans cooler should tell you something?? If its a trans cooler, the lines must begin and end at a the TRANS, right? Usually an auxilary external trans cooler is run thru the radiator first and then external cooler and then back to the trans. This is true of most factory and aftermarket setups.
Have you checked your trans fluid, perhaps there is water in the trans fluid.
BUT, MOST LIKELY If you get the radiator wet, or if it has a leak somewhere else, the fluid will cling to the sides of the radiator and run down toward the bottom, it will collect at the bottom and try drip off. It will usually start to drip first at some sort of surface anomally near the bottom. Like maybe a threaded extension sticking out of the bottom of the radiator, such as the connection for the tranny cooler line.
You may just have found water from the wet radiator dripping off the connection and it was never leaking in the first place.
The first step in finding leaks is too thoroughly clean and dry the area, and then look for the leak as it starts. Most fluids will leak and flow, then collect in a totally different low area to drip off, you can't tell where somewhere is leaking from where its dripping off. You have to clean and dry the entire area and then witness the leak at its beginning to spot the actual leak point.
