My situation was a little different, but when I was 39 weeks pregnant with my 3rd baby, I injured my pelvis really badly (long story short, I saw a toddler about to get backed over by a car, and took off in a dead sprint and felt my pubic symphysis just...*pop*...and then couldn't walk or lift my legs). I was terrified of what this would mean for my birth. My husband drove the car up next to me so I could sort of...fall into the seat, and then drove home and right up to our front door, through which he then sort of dragged me to the couch. I couldn't walk at all for several days--literally couldn't lift my legs; it was like they were cemented into the floor. I started walking with crutches about 5 or so days after it happened, and saw a chiropractor (who is trained in Webster technique and experienced with pregnancy). By the time I went into labor (40 weeks, 5 days, if I remember correctly), I was hobbling around without the crutches, but still pretty uncomfortable.
I say all this, though, to reassure you--when I went into labor, I was not moving as well as normal, but it didn't have a huge effect on how I labored. I had a really pleasant labor, 6 hours or so, and then a pretty quick birth to a 10 lb. baby. The birth was no more painful for having that injury, and it improved drastically afterwards (there was something to be said for getting the big baby off/out of my pelvis, I think).
Anyway, I'm sorry you're dealing with this--I know how painful and debilitating it can be--but I wouldn't let it give you much anxiety about the birth itself.