Given those four elements, ProFTPd, Ubuntu, pfSense, and 3.5 hours, I was able to get an FTP server working. There's a couple sticky points I'd like to note:
To get FTP to work pfSense requires you to use CARP instead of Proxy ARP for your Virtual IP. That, and you have to enable (by un-checking) the "userland FTP-Proxy application" on you WAN interface. After that you just need to add a NAT connection and rule and you should be good to go for pfSense.
ProFTPd is relatively simple to set up. Just install ProFTPd and walk through the configuration file in /etc/proftpd/proftpd.conf; it is quite well self-documented.
The whole reason for setting up the server was to give one guy access to his website, and since I have all the websites set up in one directory structure on that server (instead of putting the web sites in user directories) I had to grant that user access to his web directory.
So, first I changed the owner on the directory to that user (but left group ownership as www-data).
Then I added these lines to proftpd.conf: