This is a pretty good summary of how to check for parasitic battery drain
It'll at least narrow down where to look. I'm surprised the dealer didn't do this. Or maybe they did, I dunno.
I have a ZJ, but sometimes if your battery is low, some relays can get stuck and just keep clicking until your battery dies.
Also check your under hood light, make sure it actually goes out when you close the hood.
If parking for 3 weeks or longer, it's generally a good idea to disconnect the battery anyway.