I am on SSDI for Crohn's and have been on it for a few months now. Originally, I was on long term disability through my employer, but my employers' third party insurance company paid for a law firm to get me on federal disability. It is not an easy process and I strongly recommend getting a lawyer that is well versed in the SSDI process to handle your case. Be patient and honest throughout the whole process and realize that most people get turned down multiple times before they are approved. I was the first one that my lawyer had helped, that was accepted on the second application. Pretty much everyone is turned down the first time in order to weed out those that are trying to work the system. I was told that I would probably be turned down 3 times and then have a live hearing with a judge to discuss my case. You can probably imagine my surprise when I was only turned down once and approved on the second try.
Before you start the process, make sure that all of your doctors are on board with you. Their forms that they have to fill out really can make or break a case. Be honest about how good or bad you are. There are a ton of questions on the applications that will probably surprise you, but be open and honest and you will not have any problems. Make sure to make copies of all of the forms that you fill out and document all calls, letters, emails, and correspondence that you have with the government and your doctors. They will send you multiple forms that ask many of the same questions in different ways, and it is important that all of your answers are the same and accurate with the other forms. Plan on the process to take anywhere from 1-2 years. My process only took about 3-4 months, but that was hugely out of the norm. If you plan on it taking longer, then it will be nice if your case takes a surprising turn and is approved quickly.
Realize that many of the people that review these forms do not view Crohn's disease as a disability. Due to the lack of public awareness, we really have to make a good case of what this disease does to our bodies. Many people confuse Crohn's with I.B.S., which may be a life effecting condition, but not really comparable to Crohn's and the effects that it has on each and every day of our lives. Do not be afraid to list what your still can do, such as hobbies, because if they ever come and check up on you, you will not have to worry. I even included that I still like to camp, hunt, and fish, but have had to change the way the I do it. Good luck with it and feel free to ask any questions.