In Outlook under contacts, details, you can add user defined fields. Maybe you can use these functions in combination with a mail merge to Outlook in each of the unique letters.

Hi HJJ,

Thanks for replying to my post.
I'm not sure that this would be helpful, because the details of the contacts that I am looking to insert into these letters would just be manually entered into excel, as they are in a directory thingy online. I'm looking to print it out and snail mail.

So I've been playing around with auto text, and it looks like it could be the answer. I've tried doing Auto Text List, but I was hoping to do a friendlier format, like a drop down menu or something. Any Ideas?