lily, I really sympathise. I am 35 weeks with 2nd DC and also have it. Planning home water birth. I also have a 2 year old DS. My cousin is a surgeon and had very bad spd, and she gave me a strict talking to when I first got it at 28 weeks, and said 'look, there is now only one important thing and that is your health and that of the baby, and your future health, including pelvis, hips etc.' She really insisted that I should rest, stop work (I managed to work at home most days), and rearrange stuff round the house to cut out lifting, twisting, pulling etc. I took this advice, got DH involved basically doing everything, ignored the guilt and it got so much better, but it doesn't take much to get back to square one.
For me personally, the big things that helped were:
not commuting to work, going on bus/train, in busy crowds
not pushing DS in the buggy if possible, especially up or down hill
limiting my walking
doing pregnancy pilates and keeping a good posture
not doing anything like sweeping floor, putting washing in/out of machine
always sitting down to put socks/trousers on
shifting my whole pelvis with my hands when turning over in bed and keeping knees together
never picking up DS
encouraging DS to fetch or pick up things off the floor
not folding and unfolding the buggy (agony!)
I know it's hard to rest with a 2 year old but I just say to myself 'it's worth it as I could cause myself permanent damage and not be able to look after him or the baby, even after the birth..
best of luck, hope you can get a rest! as others have said, and I've heard water birth is very good..
I personally find the birthing ball really hurts as you have to wobble your pelvis a bit..