I resolved this to my satisfaction. I found this post which led me to a solution.
Basically, I needed to add the other IP address to the interface versus using the ProxyARP feature.
So I turned off ProxyARP and added this to my config.xml file in the system area.
<shellcmd>/sbin/ifconfig rl0 18.104.22.168 netmask 255.255.255.255 alias</shellcmd>
This works very similar to what I am used to on the Astaro firewall and it makes sense to me. I
think the documentation should be updated or we need another facility to allow multiple IP addresses
in the WebGUI to help the average user.
Thanks for your willingness to help!