I had to move my blog off Tumblr shortly after moving on when I realized I couldn't access the archive when logged out. There's no way to turn this off. And as far as I can tell, there's no way to convince it you're logged in when viewing under a domain. I was logged in, but it still showed a logged out view. Something to do with cookies, I'm sure, but this was with default security settings. I'm sure I could fix it for me, but I can't expect everyone who visits to do that.