PostgreSQL (XADataSource) as Tomcat Resource

How can I get Postgres to be deployed in Tomcat's JNDI as an
XADataSource?

I'm running my web app in Tomcat, and I'm moving towards distributed
transactions for my two data sources (JDO and JCR). I've got the Jencks
library providing JTA support (based on Geronimo's TransactionManager)
within Tomcat. Currently, I have Postgres configured as a local web-app
Resource as follows:

<Resource name="jdbc/whisper_db"
    auth="Container"
    type="javax.sql.DataSource"
    url="jdbc:postgresql://127.0.0.1:5432/my_db"
    driverClassName="org.postgresql.Driver"
    username="----------------"
    password="----------------"
    maxWait="3000"
    maxIdle="100"
    maxActive="10" />

The web-app is able to start up without errors in this case and things
function as expected. However, I've been told that I should be using an
XADataSource if I want JDO to participate in distributed transactions
(specifically, I'm trying to get 2 phase commit). When I change the
type to "javax.sql.XADataSource", the web-app fails to start because
Tomcat cannot create the JNDI entry. I'm running under Java 1.5, so to
my knowledge XADataSource is included.

Any pointers would be appreciated.

Thanks,
Mark