Success! After reviewing some of the information under the search of “SMBv1”, I played around with adding “server signing = auto” to the global section of smb.conf. I added the above parameter inside the AD container (at /var/lib/machines/nsdc/etc/samba) and now the XP hosts and the Windows Server 2003 host are able to connect into the shares.
The “auto” setting allows SMBv1.
https://www.samba.org/samba/docs/current/man-html/smb.conf.5.html#SERVERSIGNING
Now on to copying files. Appreciate the feedback.