Amy, do not use a liquid or paste stripper as the chemicals get into the drywall or plaster, don't come out, and cause future problems with paint. The instructions for these products normally tell you NOT to use them on porous surfaces. A heat gun could be used but this is very difficult work and has a good chance of allowing lead residue to be deposited all over your house which is totally unsafe.
The best way is probably to sand the walls just flush and not to try to remove all the old paint. You should take all precautions against release of old paint and dust from it. You must be totally assured that lead or lead dust can not enter your body, either while you are working on the walls now or from future activities in the house.
Personally, I would let pro painters handle the first really good repainting or I would hang drywall to encapsulate the lead paint.