Dips can be caused by joists that were cut to let plumbing, air ducts, etc. through. If not done properly, that can severely weaken a joist and make it sag. You'll need to reinforce those areas. If you jack things up, do it slowly otherwise you'll end up cracking all sorts of things. It is very possible that the footings consist of a board layed flat on the ground or a stack of stones. Get some expert help or at least advice. My guess is that it should be on-site. My unprofessional opinion.