I have resolved my dilema, but not the issue. Basically, I have changed the order of what I am doing. As mentioned above, my task consists of starting with taking an image of the updated computer, then running my cleanup script (which includes sysprep), and then taking an image of the sysprepped image. I had created a SetupComplete.cmd file that would then remove my unattend.xml file and uninstall and remove all references to the Altiris agent.
Everything is still the same except for when Altiris is being removed. I have removed the SetupComplete.cmd file and added a command within the unattend.xml file used during sysprep - FirsLogonCommands. Basically, once the local admin logs in, and prior to reaching the desktop, I am running the same batch file I created earlier to remove Altiris. After doing this, I have no problem joining the computer to the domain.
There must something about removing Altiris during the post Sysprep configuration (SetupComplete.cmd) that's interfering with the domain join process. At this point, though, I'm fine with how I've worked around the issue.
Thank you.
Eddy