Are you running a config of JGroups with flow control (FC) ? Try fc-fast-minimalthreads.xml from the JGroups src code of the version you are running.
I couldn't find a copy of that configuration anywhere. I did find another minimalthreads configuration xml file and noted that the up_thread and down_thread parameters seem to be the change. I've tried this, and it does seem to have stopped the leak we were seeing. However, we now have PingSender and PingWaiter threads leaking out at an even faster rate. Could you post a link to where I can get that configuration file to try?. I'm going to try setting the PING protocol threads back to true and see if that helps.