I sympathise fully. I've been in agony with SPD for about 3-4 weeks now and am now 21 weeks (so not as far along as you). My midwife has referred me to Pyhsio, which hasn't been much help - but I'm hoping to get some hydrotherapy in, which may help. I've been seeing an osteopath though - which has really helped but the pain is still there.
I know I'm a good few weeks behind you yet however this is the advice my osteopath gave me:
Avoid crossing your legs (not that you'll be doing much of that at the mo)
Avoid sitting with legs apart
When sitting, sit with both feet on the ground
When getting into the car - try to sit in bum first and then swing your legs in together
Use a pillow at night between your legs.
If you do swim - don't do breaststroke.
I've found sitting upright in a supportive chair - not on the sofa helps massively. There are also exercises you can do on a gym ball which may help.
I've also just ordered a pregnancy support belt - but not used it much yet.
Rest - move positions frequently, don't stand up for long periods and get someone else to cook and hoover!!
On the plus side it does seem that there is no reason why you can't have a normal birth. I want a home birth and providing there are no other complications my midwives are happy. You just need to make sure that they know you have SPD (or PGP as it's now known) and that there is only so far you can open your legs.
Someone else gave me this link and my physio printed me out the leaflet. It's got some good info in it.
www.acpwh.org.uk/docs/ACPWH-PGP_Pat.pdf
I hope all that helps and sorry for the massive post.
Look after yourself - it's horrid!!