Hi, congratulations on your pregnancy !
I don´t have any personal experience with breastfeeding on Imuran, but I do follow a thread of Crohn ´s patients on pregnancy here (Czech Republic) and all were advised to stop taking Imuran with the end of pregnancy in order to breastfeed.
I myself came here to this forum not long ago with a question about
taking Entocort when breastfeeding
. Decided not to risk side effects on the baby, stopped taking E. and have been breastfeeding my daughter for almost 3 months - I am lucky, tests are fine and we can carry on. I was prepared to stop breastfeeding the moment I would feel the illness is coming back though and give baby formula- my son couldn ´t have been breastfed and he is a healthy boy now.
Hope you make a decision you will feel happy with and enjoy you baby, good luck !