Dog is scared of people- can any behavior experts give me advice? (see details)

The boy and I adopted a 2 year old spayed female lab/hound mix about 3 months ago. We know nothing about her background other than she was at the shelter for at least a month and was very, very skinny when we got her.

She was skittish and disinterested in us when we adopted her, but we’re suckers and she came home with us. Most things are 100% perfect. She’s super attached to me and the boy, and rarely goes more than 15 or so feet away, even off leash. She likes other dogs but doesn’t seem to know what to do with them, so one day a week she goes to doggie day care to learn how to be a dog. So that’s her background, and here’s the issue:

When we got her she was indifferent to strangers. Now, she’s flat out afraid of anyone who shows her any interest (aside from me and the boy). She’ll sniff people, but as soon as anyone turns toward to her, tries to give her a treat, tries to gently meet her, she bolts. This includes people she’s around a lot and who have never been aggressive, loud, or whatever in her presence.
So, if anyone out there is a dog behavior expert and can give me some advice on how to help my lil friend out with her fears, it would be very appreciated.