These are my thoughts - they could be all wet as I haven't seen the dog in action, nor witnessed your training so far.
First, what is her threshold? It sounds like you know the distance when this happens -- work UNDER threshold - ie, turn around before she reaches this point.
Second, if you have used an electric on her previously, it is a little bit late to go to positive only... in that you have brought the fight out in the dog probably with the ecollar.
Try a flat leather collar if you want to move away from what you were using previously.
Third, is it true that if someone else has her on lead this behavior does not happen? If so, video how they handle her. Then video how you handle her in the same situation. See how your handling is different than the other persons. It will probably tell you a lot.
Fourth, Do the other people that she does not grip (I'm going to call it a grip because you haven't mentioned puncture wounds or needing to visit the er) walk the same route?
And another question - what happens if you keep walking? Dog attached to your arm, you keep going. Forget luring her or anything and just keep walking?
does she do this on every route you take?
what if you drive her away from the neighborhood and walk her? How long does she go without this behavior?