I'm sorry to read how your urge has affected you. Has your urge changed in any way since you first posted?
For me personally, my ankles inflict the highest cracking urges, too. I could "unlearn" my clearly obsessive and unhealthy popping urge for most joints, but the ankles (and toes) are still persisting and thus the hardest to reduce to "normal" levels by far. For ankles specifically I learned that regular jogging helps, due to the repetitive (non cracking) movement.