I don't believe you can use ESB server for that. The ESB server is stripped down and some components have been removed (for example, EJBs) to provide it with a quick start time. I believe that some of the components that were stripped out are necessary for clustering. Additionally, you wouldn't really want to do that - the ESB server is not for production use - it's a stripped down server that was intended to start quickly for development.
If you have it working on AS 5.1.0.GA, you've done all the work, I'd stick there.
Thank you for replying me. I will go with AS 5.1.0.GA.