So awesome of you to rescue this wonderful breed of dog, he is probably very thankful.
My 2 gsd are very vocal dogs and from what I know, many gsd share this trait. There is a large possibility you will just have to get use to it. I have no suggestions as to how to correct this behavior. It sounds like when one of you goes into a store he is showing a bit of anxiety, perhaps when someone goes in, they could bring him a goodie when they come back?
As for balls, yes gsd LOVE fetch....but many of them have very powerful jaws and unfortunately, destroy alot of toys in a couple chomps. My female is batty for tennis balls, they are easily her favorite toy in the world, however in a few chomps, she pops them! I found a great alternative toy for fetch is a large Wubba (made by kong, but nothing like a kong). If your dog has really strong jaws, like my boy....perhaps you should invest in some orbee toys. My boy can destroy ANYTHING (all those 'indestructables' last him but a few minutes) the only toy he has not been able to wreck is his strawberry and artichoke, made by orbee. They are wonderful toys and last for ages.....plus they are really cute and smell like mint!
I also found the chuckit flying squirrel to be a great success with both of my dogs.....they loved that thing. Too bad it was destroyed in less then a week.