My Lab had terrible separation anxiety from around 9 months old, hers was displayed by mass destruction of my house. Once I'd identified that it was SA it was resolved within around six weeks or so.
Firstly I got a DAP defuser. DAP stands for Dog Appeasing Pheromones and the diffuser, looks very much like a plug in air freshener, give out synthetic replica of the pheromones bitches give out just as they've had pups. This helps give dogs a "sense of wellbeing" and can comfort them. You need to have it plugged in 24/7 for at least 3 months I would say for SA, thats about the time I had it in for. Obviously if she hasn't started to show signs of improvement keep on longer, or if you turn it off and she re-greases keep it plugged in for a prolonged period. You can get DAP defusers from your vets and online. This is ones of the cheapest online suppliers I've seen
http://www.vetuk.co.uk/index.php?main_page=index&cPath=149_2You'll see they have a DAP spray and also a collar, which is a brand new product, but for SA I'd use the defuser.
I also used herbal drops for anxiety, which I would drop into her mouth a few minutes before I left. There are a number available, the actual one I used isn't available now. However I have used Company of Animals Anxiety Drops for other things since and they've worked well, also Dorwest do Valarian Compound Drops both of these are available from the same website as above.
http://www.vetuk.co.uk/index.php?main_page=product_info&products_id=666 and
http://www.vetuk.co.uk/index.php?main_page=product_info&products_id=472Dorwest do a Scullcap and Valarian Tablets which you can buy directly from them, these are a registered herbal medicine where as the drops are classed as a dietary supplement. http://www.dorwest.com/new_pop_ups/scullcap_valerian_taps_vet.htm
After a couple of months I stopped using the drops, I'd play it by ear though.
I know you said you've left her with chews when you go out. I used various activity toys, for example Kongs, Squirrel Dude, Twist & Treat, most of which are available from
http://www.dog-toy.co.uk/dogproducts/?page=3 The big difference was that she only ever got one of these things when I went out, so in the end she got more excited at the prospect of getting one of these when she saw them come out and her focus was very much on that and not on me leaving.
Once she gets adept at using the Kong there are various ways of filling them and freezing or microwaving them to prolong her entertainment.
Another thing you could leave her with is the Buster Cube.
http://www.dog-games.co.uk/newshop/product/Buster+Cube+treat+toy+-+Large+and+Small+sizes.htmAt the time I was told to limit how much access she had to rooms in the house. Apparently sometimes dogs can fret if they feel they have too much space that they have to "look after", so we limited her to our entrance hall whilst we were addressing this anxiety. A crate may be the answer for some, I am a crate user, however I do know that my cocker creates much more when left in his crate because he thinks he's missing out on something.

So for him leaving him out of the crate resolved that.
As you have a crate I'd leave it in the room you are going to limit her to, leave it open and cover three sides of it with a blanket, just leaving the front open, to create a den for her that she may feel safe in.
Sapphi was my first pup, and her separation anxiety was down to me. I'm at home all the time, rarely going out, I only ever went and did my housework etc when she'd fallen asleep, when she was awake I'd sit and play with her. Therefore she didn't learn about being on her own and occupying herself. So as well as working on the things I've stated, I began to put some distance between us, shutting doors so she couldn't follow me. Just for very short periods at first and I built up.
I also left the TV or radio on which can really help too.
Having learnt my lessons from this, I worked hard at not falling into the same trap with my subsequent pups. They are all happy to be left on their own. I didn't get my second pup until I'd resolved this with Sapphi.
I don't necessarily think that a companion will make a difference. As already has been said there is every possibility that she will transfer her anxiety on to her companion. I have recently looked after a friends two cocker's, they both have SA to varying degrees. Interestingly they used to being left for a few hours a day while she is at work, they have companionship from each other yet they do suffer with SA. So just adding a companion isn't the answer.
Hope this helps a little.