Hi,
There's a lot of detail to be known to help you. I wrote a blog entry which discusses MS Index Service use when IIS and SQL are on different Servers, including the use of Medium Trust. It was written when I was using DMX3, but it still applies with DMX 4.
https://www.silverbullettech.com/TechBlog/tabid/94/EntryID/5/Default.aspx
See if it helps you get further.
Rob Ralston