If you have a heavy messaging load, and find that your messages are accumulating unevenly across a distributed destination, then one possible solution may be to set "affinity" to false and "load balance" to true on your producer connection factory.
There's detailed documentation about the producer load balancing algorithm in the Load Balancing Messages Across a Distributed Destination section of the JMS configuration guide's "advanced" chapter. See Configuring Advanced JMS System Resources - 11g Release 1 (10.3.6). Also, I think the JMS chapter of the book "Professional WebLogic" has a fairly nice treatment of WL JMS load balancing in general.
Hope this helps,