Is there any way I can get the app to
work on JBoss 5 without making any
code changes? Config changes are OK.
Assuming that your code is written to the JMS API, then yes, you probably can. However, don't underestimate how different JBossMQ is from JBossMessaging, the configuration is completely different.
Depending on how hard your app pushes the JMS API, you may get behavioural differences, but the odds are you'll be fine, as long as you can reconfigure JBossMessages within JBoss5 appropriately.
Another option is to move to JBoss 4.2, which still has JBossMQ, but is miles ahead of JBoss 3.2. Much of the functionality that JBoss5 gives you can be done with JBoss 4 (e.g. it has partial JavaEE 5 support).