It's a classpath issue, it looks. Not sure why it couldn't be loaded.
You've got it on the extensions classpath. Is there any way you could repackage the application and place this file in the lib directory of the war? What about moving the file to the WebSphere \lib\app directory?
Here's a tutorial on understanding WebSphere and
J2EE classloaders. Maybe it'll give you some insight into your problem:
Demystifying WebSphere Classloaders: Loading Java Classes at Runtime
Good luck!
-Cameron McKenzie