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Long term SI issues (w other stuff/chronic pain), for the SI joint physical therapy used to be enough. I've found bouncing/rocking on a ball to really help (20-30 min a day in a single session). I also use lidocaine patches with the blessing of midwife which helps so much more than just tylenol (I can usually use instead of tylenol for SI stuff). I used an SI belt early in pregnancy, now I use either a 6in ace bandage wrapped tight or one of those heavier duty maternity belts like this one. And ice it whenever I can (strapped into the ace bandage for security)
Maternity Belt - NEOtech Care Brand - Pregnancy Support - Waist/Back/Abdomen Band, Belly Brace (White, Size S) [link]
I have the Neotech belly belt and it's wonderful. I wear it if I'm going to be standing or walking for more than 30 minutes (cooking, running errands, etc.) and it's made a huge difference. My lower back is still really tired at the end of the day but I'm in a lot less pain.
I'm not sure if this is an option, given your herniated disc, but have you ever seen a chiropractor? I started seeing one a few weeks ago and that has been really helpful as well.