I found a few things on Minti for those who have SPD... and there have been a couple of members who have had it. Check out those articles I've put in the related content links.

I've had this with my last 3 pregnancies, and dreading getting it this time round.  Yours sounds extreme and that there is the possibility of an early labour.  Listen to what the docs say and rest as much as you can.  As for the baby going down at 36 weeks, mine usually go head down and stay that way around 25 weeks, and drop further around 30 weeks.  Then for some reason, around 34 weeks, they are sitting up high again.  I hope things work out and that you don't end up into too much pain from it.

Sorry, I don't know anything about this but I just googled it and came up with this hope it helps.