Glad to hear you are feeling better, and that you have ditched your walker! That's fantastic! I know how long you have been using the darned thing. I am thrilled that you can sleep on both sides. I still have trouble with the surgical side after all these months. I found that decreasing my opioid pain meds definitely had an affect on my mood, but for me it was more that it made me cranky and irritable. Decreasing those pain meds can be problematic, as we become physically dependent on them after a week or two, no getting around that, and when we decrease them there are side-effects.
I googled the sacroiliac joint, and it is not next to the tail bone. They are above it somewhat, so your pain must be something else. Wish you knew what it was! I hate being in the dark about things like that.