Proxying server setup help

[Antony den Dulk]

I have the following setup:
- an internal server with IIS that talks with Tomcat - Shibboleth is
installed in IIS - https://internal.domain.com
- an external server that proxies to the internal server -
https://external.domain.com

The external server is used to proxy to various internal servers and
distinguishes the various servers using paths.
- https://external.domain.com/division1/app1 -> https://external1.domain.com
- https://external.domain.com/division1/app2 -> https://external2.domain.com
- https://external.domain.com/division2/app1 -> https://external3.domain.com

I am trying to set this up so that Shibboleth sends the correct URI in
the Authentication request, something like
https://external.domain.com/devision/app/Shibboleth.sso/SAML2/POST.
Instead I am getting a URI in the form of
https://external.domain.com/Shibboleth.sso/SAML2/POST.

I need the internal server to listen on
https://internal.domain.com/Shibboleth.sso/SAML2/POST since the
external proxy strips the division/app part of the URI from its
request.

What solution(s) are available to achieve the required result?

Thanks,
Antony