Imuran can cause severe bone marrow depression (among other things). Get monthly blood work.
I was at 200 mg Imuran/day before I noticed anyting--that may be too much for you.
Remicade may not be the next step, there are other treatment options. Many here don't like it but have you tried prednisone to get you into remission?
Once in remission you need a maintenance drug to keep you there.
Also, instead of sulfasalazine you should consider a 5-ASA drug like Mesalamine.