We finally got it figured out. I definitely needed to upgrade both Snaps to OS 4.0.860. Then we followed the instructions from this link:
http://wiki.procooling.com/index.php...able_.5Bfix.5D
but I still had access issues, so I turned off Novell Networking, and then had to refresh the Workgroup/Domain user list from the AD2003 server by using my network ID and password as an example (in Security > Security Guides > "Using Windows domain security").
That did the trick.
Thanks for everyone else's suggestions... much appreciated!
PS: We access these servers from the PCs using a shortcut that was originally created by going to My Network Places > Entire Network > Microsoft Windows Network > [workgroup name] > and then both Snaps are listed as "Snap Server 172.18.61.5 (GSserver3)" and "Snap Server 172.18.61.6 (GSserver4)". From the Mac side, we never had a problem accessing them using just the IP address using Go > Connect to server... in Mac OS X (10.4.11).