After deep looking through the jboss and TorqueBox files I was not able to solve the issue. Fortunately, I've got some help from torque box - IRC Channel.
If you want to use db2 with WEBRICK you need to copy db2jcc.jar and db2jcc_license_cu.jar to jruby/lib (~/.rvm/rubies/jruby-*/lib) directory.
The good thing about TorqueBox is that
TorqueBox automatically adds any jar files under the applications lib/ directory to the classpath of your application
So, just copy these files into your application lib/ directory.