Joint pain and arthritis are definitely symptoms of IBD. However, I will say, that sometimes the medication can cause it. For example, when I was on Remicade, a couple months into taking it I developed horrific joint pain. To the point that I would be in tears it was so bad. Then when I got off Remicade, the pain left. I'm not saying it is the Imuran causing it, but that could be a possibility as well.
In the meantime, have you tried Epsom Salt baths? It helps relieve some of the pain.
Big hugs, Elizabeth (team member)