If you don't have access to a machine, you might want to try pull-up bands. They will allow you to do the pull-up, but you can reduce the weight you have to pull by using the bands. Here are the kind I'm looking into getting, RubberBanditz. I've been using a machine to allow me to build up my strength as I go for the unassisted pull-up, and I have no doubt you'll be able to build your muscles up with these, but PLEASE check with your doctor to make sure that pull-up are okay. I'd be afraid to just go for it if I had the medical concerns that you do.
Anyway, I've gone from 120 pounds of assist on the machine to 75, though it's taken me more than six months. If your doctor gives the okay, I know it will help strengthen your muscles to pull up with a bit of assistance. I don't know why people are so down on using a machine. I don't see it as any different than using a leg assist, which Mark recommends. But there I go again, thinking independently. We can't have that!
Motherhood: When changing from pj pants to yoga pants qualifies as 'getting dressed'.