I have a sulfa allergy and was given Asacol (though was also first diagnosed as UC). I think there is a drug like that which is non-sulfa and absorbs earlier than Asacol, which doesn't break down until the large intestine. I suppose it depends upon where your Crohn's issues are.
And, though it didn't work for me, Asacol helps many, many people! I have a friend with Crohn's who maintains remission using Asacol and the same with my aunt who has UC!
Also, if you are bad enough, I recommend asking your doctor about Remicade. It seems scary to people at first, but it did wonders for me and there are no purse-loads of pills to carry about on a daily basis!
Good luck!