Anxiety and aggression

Hi,

I recently adopted a 3 yr old neutered female JRT. She's had kind of a rough start (first owner let her run amok in the house, and second owner's answer to her problems was to crate her for 20 hours a day) and I know she will be alot of work. Separation anxiety makes her cry and scream (although only for about 5 seconds after I actually leave the house) follow me very closely and try to run out of gates and doors ahead of me. I am working on this problem by leaving and returning throught different doors of the house, and trying to leave without her noticing (actually not terribly difficult). Today, however, I took her in the car with me and she bounced around, cried, screamed and barked the entire length of the car trip, creating a dangerous situation for both of us. When we brought her home with our other 2 dogs (neutered M Cairn terriers), on a 3 hour car trip, she was an angel. What should I do to help her relax in the car when it's just the two of us?