Thanks for the suggestion, unfortunately that will not solve my
problem.
I cannot build user authentication into the application itself, so I
need a 3rd party method of restricting access to certain pages. The GET
method using URL parameters is the only thing that differentiates one
page from another, so hiding them would only make life more difficult.

Most authentication software (IIS Protect, Authentix, etc) uses the
directory structure or specific file name to restrict user access. In
my case thats innapropriate as I have one file and one directory
structure.

I need an authentication software that can identify URL parameters as
well as files and directory structure.