I know it's kinda obvious, but have you tried any stop leak? I've had good luck with Bars stopleak, cause that's what my buddies have at their shop for free.
I had a pretty good leak going, and guessed it was an intake leak on my old 350 burban and had good luck with Justice Brothers Radiator Stop Leak. Read and follow the directions, 'cause I didn't and had to do it again. I drove the old 'burb another 20k miles and ended up selling it (reads: new wife no drivey-drive rusty burbans). After I used that stuff I did not use coolant in bulk again, but remembered to warn the new owner about it. I haven't tried it in my jeep yet as I don't drive it daily but probably should listen to my own advice...but adding water/coolant is cheap, especially when it's free <img src="/forums/images/graemlins/smile.gif" alt="" />