I use IBM Informix JDBC 2.21.JC5 with JBoss-3.2.2RC2.
I did not encounter any problems when upgrading from JC3.
What problems do you get ?
I will have to rerun the server and get this information to you. Our group uses DBVisualizer 3.3.1 as well, and I cannot get this to work with the Informix Drivers version 2.2.1 JC4/JC5. It works great with JC3.
I'll reply again with the errors I get in the server.