Sorry to butt in here but I seen the title of the post so I thought I would share
I too have Sacroiilitis but I wasn't a clear cut case. I had both xrays and MRI's both of which came out normal. But whenever the doc would press on my SI joint, it would send me thru the roof. I had 2 steroid injections into my SI joint and Ides is right. They use an xray guided machine to get into the SI joint. It isn't a pleasant procedure but it does help.
You mentioned that you have been on steroids from your CD. Have you ever had a bone scan done? If not, then I recommend you get one. Long term steroid use has the potential of causing bone necrosis.