It could be either. What year XJ do you have?
Check your hoses and see if they are in good condition and secured tightly, if the hose is cracked or bulging, replace it. Bear in mind that your overflow tank may be right around that spot as well and that your heater hoses are also on the battery side.

You can rent a coolant pressure test kit from your local parts retailer that will allow you to pressurize the system manually. Do this and you should see coolant spraying out of wherever it is leaking. Replace the parts and re-test until the system holds pressure.

My money would be on the radiator seam (the part between the metal fin part and the plastic tank) has failed or that your overflow tank cracked or the hose has come off it.

If you're replacing hoses or the radiator, flush your system while you've got it apart. If you have one bad hose, replace both them as a pair - otherwise you're likely to be getting covered in coolant a second time in the near future.

 

preferably clean the area

run truck to operating temp, open hood, stop engine, jump out and have a listen, as pressure builds for a fews secs after switch off,
and you can often hear a hiss

follow the coolant trail thru the previously cleaned area