A reputable doggy day care is an excellent suggestion. Ask tons of questions & visit lots before joining. And like mentioned some offer seperated groups & maybe they can call you if a problem occurs or she is under too musch stress & you can pick her up right away. And yes I would start off by leaving her for just a couple hours, then work your way up to a full day. If you know no one that has big dogs that is about your only choice to get her used to big dogs.

It sounds as if she has a fear of riding in cars or what not, as seperation anxiety iss just that, anxiety that happens when your seperated, since you are together in the car it sounds as if she has a different anxiety. Does she go elsewhere besides pet smart? I would take her everywhere possible w/ you, even while running errands. If she is very very very nervous in the car then I would start off slow, just sit w/ her in the car in your driveway IGNORE HER WHEN SHE ACTS ANXIOUS, PRAISE WHEN SHE IS CALM ALWAYS!!!! Then sart up the car for a few days, not going anywhere just sitting in the driveway, then go to the end of the driveway, then around the block, etc... etc... When she starts to improve then move to the next step. If she regresses then go back to the last step & start again from there. Also try to make the outings fun, drive down the street to park to take her for walks instead of leaving from your house, go to the park. Alwyas remeber to praise when she is calm.

I wish you guys the best of luck!!!