I wasn't diagnosed with it, but since finding out about it I'm certain I had it. Have you talked to your OB/midwife about treatment options? Physical therapy, acupuncture, massage, chiropractics all may help. Even just icing it may help (again- talk to your OB/midwife).
I didn't try it myself, but in hindsight, getting to the pool may help a little bit. The water takes a lot of the weight off your lower body. I believe they still have pregnancy exercise classes in pools for that reason, you could give that a try or just free swim/float.
If you don't have one, look into a massage pad. We got a great one at good will- the whole back and seat were heated, you can turn specific massagers on and off (upper back, middle back, lower back, and two on the seat). I practically lived on it the last trimester. It got ruined by mold and I'm definitely going to get one before next time. (okay- maybe the muscle pain for me was unrelated to SPD but it still might help)
this is just a moment in time, step aside and let it happen