It's in okay shape. It's old corroded cast iron, but no breakages/etc. I think if we're excavating (with the full length being 18' and the problematic section being 9'), its probably worth doing it all for future prevention.
Right now we're actively avoiding feminine hygiene products, and watching it. If it keeps happening we're going to pull the trigger, but if that alleviates the issue in the interim, this will go on the back burner for a bit. Unfortunately there's no quick way to find out (short of taking a few big bowel movements and hoping I dont have to shopvac the basement again..)
edit: I'm just going to setup a burn barrel in the back yard and insist we no longer use the bathrooms in the house