In case anyone reads this old post, I went off Immuran for 2 weeks. Symptoms kicked up, which wasn't great, but white count improved. They put me back on Immuran at a slightly lower dose and so far, things seem to be going well. I retest again in about 2 months to make sure counts are holding okay at this lower dose, but the good news is symptom management is back again, even at this lower dose. I had been on a diet and losing weight, so being at a lower weight I probably needed less of the drug anyway, plus I had probably been restricting my calories a bit too much (to meet a diet "goal") and caloric restriction can also lower one's white count, so that also didn't help.
Happy to be back on track with my trusty Immuran/rotating antibotics combo for as long as I can ride this out!