Ant advice on how to react?

Hi! My husband and I recently got a puppy, she is 19 weeks old. So far, she has been a GREAT puppy. We picked her because she is very calm, sweet and pretty submissive. We have only had her for a week. She is a lab mix, probably with mastiff, but she is not a huge girl, about 35 lbs at 19 weeks. We have two kids, aged 6 and 2, and she doesn't chase them or nip at them or anything. She does lick them, so we are teaching them to say, "no lick" when she tries to lick them, and this has been working. She listens pretty well for a puppy, and is even learning not to chase the cat. When she has chased the cat, ashe has respected his "hiss" and is very gentle with him.

We are not letting her on the furniture, and we are taking measures to make sure we remain the "alphas" in the house, like making her wait to eat, and making sure all the kids and adults go through a door before her. She knows the word "No" pretty well. She knows to take treats from our hands gently, and likes to play ball (sometimes). I spend time training her everyday, and she does pretty well, so long as I have the treats with me. She is great with all the neighborhood kids when they come to play with her. She is crate trained, and pretty well house broke.

Here is the problem I am having with her: she doesn't like to walk, when I put on her leash to take her on a short walk in the morning and at night, she doesn't want to go. Sometimes, she will decide to come along, but often it is like pulling teeth. I (right now) read not to force her, so I think I will start bringing treats along to encourage her instead of just tugging her down the road on occasion. When I am walking her in the early morning and evening, she is what I call "on alert" and seems edgy, maybe a little scared (but not submissive scared, KWIM). Her ears are pricked forward, and she keeps her head up. Sometimes, we will encounter a neighbor while on these walks, and when the person bends down to pet her and calls her, she will walk/run forward pretty submissively and give them some love and they will pet her. Today, while I was walking her she was pretty much on alert, but walking OK when one of my neighbors came out to go to work, Hoku (my dog) stood on alert and barked at her, raising her hackles. I tried to encourage her to approach my neighbor, but she refused, and my neighbor was in a rush to get to work, and is not a particular dog lover, so I didn't push the introduction. My neighbor was across the street.

I AM NOT CERTAIN what to do when she behaves this way! Do I reprimand her for acting in an aggressive manner? Do I try to reassure her ( I am afraid of inadvertentaly encouraging this her aggression)? Usually, I would ask the person to pet her, but I couldn't in this case, as my neighbor was clearly in a hurry to catch the bus. Also, when I walk her in the afternoons when all the kids and people are outside, she seems more relaxed, and is not "on alert."

I do not want an aggressive dog, and we picked her because her previous owner said she was the most laid back and sweet puppy ever (which has been true). She did say Hoku was bitten once at a dog park, and Hoku does seem a little unsure of unkown dogs. However, once she has overcome her initial fear, she has played well with the other dogs in our neighborhood.

Finally, it turns out her shots were not kept up to date, so we can't take her out and about too much until she has had all her boosters (which won't be for 4 weeks). So, I can't enroll her in a puppy/dog training class for at least another month either. Our vet said it is OK to walk her on our dead end street, and to let her play with the neighbor dogs who we know are vaccinated.

Anyways, any advice on how to deal with her little show of aggression (and to nip it in the bud) would be greatly appreciated.