Admin-Ajax.php, SSL, Non-SSL

I would recommend to define this constant in your wp-config.php to force HTTPS on admin:

define('FORCE_SSL_ADMIN', true);

Also, there is a function called is_admin() which could be helpful in your case.

if ( is_admin() ) {
    $_SERVER['HTTPS'] = 'on';
}

However, if you have a rule in your web server forcing all wp-admin and wp-login.php requests to run over SSL, you will need to add an exception for admin-ajax.php file. Not sure how to implement this in Apache, but for NGINX the easiest way would be something like:

if ($request_uri !~* "^/wp-admin/admin-ajax\.php") { rewrite "^/wp-(admin|login)" https://$host$request_uri permanent; }

Hope this helps!