You're overheating, most likely, because you're losing pressure from a leak... Firstly, when you replace the coolant, are you using a 50/50 mix of coolant and water? Straight coolant is not a good cooling agent at all...
If the radiator's cracked, could it be fixed, technically yes, but it's probably at the point where it's gonna keep getting more and more and more leaks, so I would just replace the entire thing, if I was you...
Are you sure it's the radiator? Check the hoses where they go in to see if they're cracked, if memory serves me correctly, not looking at mine right now, obviously, but I want to say that the lower hose goes in on the drivers side, and the upper hose, the passenger... If anything, start it and get it up to pressure, you will be able to see much better where specifically it's leaking from, just DON'T touch the fan
If not radiator, front of the engine area could also be water pump, but based on the amount of leakage, I don't think that's the case, don't discount it though... Like I said, just get a definative idea of where it's leaking from; radiators are not that hard to do really (though I've never done one on an XJ...) '90 Bonneville, 3 feet of snow on the ground, working in a driveway, had it done in about 2 hours (that's including three cocoa breaks...)
Welcome to Jeepz, hope you get this problem fixed, and stick around
