403 error when making a service call

403 error when making a service call

I've got a weird IIS issue I'm trying to work through.
We have an auth service running in our dev environment. When called inside
our VPN, just on a browser via HTTP request, the auth service works fine.
But, when I try to make the exact same service call from a website running
on the same server as the auth service, I get a 403.
When I hit the URL directly, I get the correct xml response.
When I hit the exact same service call, IIS spits this into the log: -->
403 4 5 0
I'm kinda stumped on this. I'm wondering if the error might be because, in
this case, the website making the service call and the auth website are
different applications on the same website, and there's some kind of
cross-auth error. I'm not sure.
When I make the service call in my dev website, I do fully qualify the
domain. I make this same service call from a different website, even a
different server, and have no problems. Again, naked request on a browser
works fine too.
Thanks in advance!