Here is a little background on my situation. My ex and I have been divorced for 10 years. She had custody of our 2 son's for the first 2 years, then after I remarried my wife and I took her to court and got custody of my oldest (which was 10 at the time) because he cried every time I took him home because he didn't want to go to his mom's he wanted to stay with me and my new wife. Even though I have custody of one and she has custody of one I still pay child support every month.. And have always paid her directly.. Never going through the child support enforcement.
Now my oldest, is turning 18 next month... and yesterday I received a letter from the child support enforcement division telling me that certain things have changed and that I might need my child support revaluated. Since we have never used the child support division. Does anyone think that maybe my ex had turned me in?
My other question is if I file for joint custody do I have a chance of getting it for my youngest son.

I am not sure what you mean by turned you in, but I do hope you can prove you paid her all this time. As for custody, it is the best interest of the child. It is going to come down to the facts of your situation and the judge that you go before. I would suggest that you speak with an attorney to discuss your legal options. You can locate one through the attorney locator on Lawinfo's home page at http://www.lawinfo.com/. Some do offer free consultations.