Is it okay to take imodium (loperamide) while on opioid painkillers?

Yes: Imodium's action is primary on direct contact with the gut, and not very much is absorbed, so it will work even if you are on other opioids. Follow label directions and do not exceed. You may, however, have better luck with Bismuth Subsalicylate preparations (pepto bismol). If the diarrhea is chronic, you need to look for a cause and treat that directly - like irritable bowel syndrome and others.