Are you getting the ExceptionInInitializer error, or is it just not responding at all?

If it is simply not responding, I would suspect that it's having a problem locating the MRJ in the correct System Folder, especially given that you experienced the problem when running OSX in Classic mode. You might want to try uninstalling and reinstalling the MRJ again, but this time making sure that its added to the System Folder that whichever OS you're currently using is running from.

Also, if you're using OSX, I would recommend just giving the OSX version a shot. It is very strong, and you don't need to worry about the MRJ at all. It will also be improving significantly in the future (file and URL launching will be added, for example), and is definitely the preferred Mac version of LimeWire over the long term.