Hiding Evidence

I live in Southeast Texas. I am currently involved in a discrimination lawsuit and I have no representation. I was about to settle the matter last week when I discovered that the defendant is in posession of a document that will prove my case. They received the document in early January and they chose to try and conceal it. They pushed me to settle the matter and I was prepared to do so until I found out about the document in question. It helps my case tremendously and they know it so they tried to hide it from me. My question is- Can they legally conceal this evidence from me ? I have asked for a copy of the document and they have refused to provide one. What can I do ?

Assuming this is a Civil matter, they are not under the same rules of evidence as in a criminal case. They do not have to make their documents available to you upon your general request.

If you want a copy of the document, you need to subpeona it. That will then force them to turn over the document to you. I would suggest that you make a broadbased request... to the effect of:
"All documents relating to the hiring, termination and employment of "X" that are in your possession."
(This is a very simple example of the type of request and documents, recording, emails, etc. that you need to include in your discovery.). I suggest you take a look at the Rule 177a of the TX Rules of Civil Procedure at:
http://www.adminlaw.org/trcp29.htm#177a