the same URL works both internally and externally on my sbs2003 system:
(In my case, the x and y are tzo.com because I used to not be on a fixed IP
service, and my self-signed certificate is set to this name.)
Whether or not this works depends on the router hardware connecting you to
the Internet. Some routers will handle such a request from the LAN side in
the way you expect, and others will not.
Russ's solutions look much more comprehensive, and probably work around this
issue of the router--but for me, this works without any effort.