Although there are longterm side effects of Imuran, there are also harmful side effects of long term prednisone use as well. If you need to go on Imuran in order to get you off prednisone, I would say go for it. I do have to disagree with Howright however, some people are able to stop taking Imuran without tapering, but others actually do need to taper much like prednisone. I think it depends on the person.
In my case, I am allergic to Imuran and unable to tolerate it. A couple weeks after taking it I became jaundiced and went into organ failure. It was simply too toxic of a drug for me.
best of luck to you though! I hope it works for you.