After quite a bit of poking around, I discovered the cause of the problem. In the user’s profile in AD, the Account tab has a Log On To button where you can add the names of computers where the user is allowed to login. Accounts that didn’t work with 10.5.4 clients happened to have an entry that allowed them to login to the XServe. After I removed all entries and selected the option for the user to be able to login to all computers, the volume automatically mounted when the user logged into a 10.5.4 client and mounted a volume on a 10.4.11 XServe.