Help with aggressive barking

[Kerryh1986]

Hi,

I have a 2 year old springer/staff cross. She is a lovely dog in the home, generally calm and loving.
Once in the garden, she becomes really aggressive if anyone so much as walks past the gate. She barks aggressively and jumps up at the gate and fence, which is obviously a scary experience for the passer by, and is causing problems with neighbours. She doesn't respond to any stop commands, and only stops once the person has passed.
I am fairly confident that this is down to nerves, but I have no idea how to ease that and put an end to her behaviour.
She is really nervous with new people, which worries me. She was spayed around 3 weeks ago, in the hope that this would help. but it hasn't. I was very concerned with how she would be once left at the vets, as I'm really not sure if she would actually go so far as to bite someone. Although she was very nervous, there was no sign of aggression.
I've been looking online for solutions, the one that crops up the most being an anti bark collar, but I think this would only aid her fear.
The other thing I've seen is the thunderjacket. Does anyone have any experience of this?
If anyone could help with any ideas at all I would be very grateful.