I have UC not Crohns but i've been down that path with what meds are safe etc..

I spent both pregnancies on 25mg of pred. The thing to remember is A flare up during pregnancy is a lot worse for mother & baby.
I was told Imuran is okay during pregnancy & breastfeeding. That will just monitor you more closely.
I know of a few woman who have b/f while taking imuran, most of them pump & dump.
Pred is also safe to take during pregnancy & breastfeeding.
I think if you dont feel safe taking Imuran then your best to start taking the pred, I know it's horrible & all but a flare up will be such much worse for you & you know that will happen if you stop taking your meds.

I had a minor flare at the begining of my 1st pregnancy & then stayed in remission till 8 weeks after giving birth. My 2nd pregnancy i flared about 20wks then stayed in remission till 10 weeks after giving birth.
If you have any more questions i'll be happy to try answer them