Golden Lab? You mean Yellow Lab I think
I agree with Slannesh. You need to look at this dog's diet, forget the dandruff shampoo. Make sure it is on a good food (I would stay away from grocery store brands). Try supplementing the dog's food for coat and skin. It simply sounds very dry. I used ground flaxseed in my dog's food for a while when I noticed her skin was dry from the winter.
If bettering the diet doesn't work, you might run to run it by your vet. Not only is it unsightly but can be uncomfortable for the dog. It sounds like you've just noticed this, so it can probably be helped. I know my cat always has had dry skin, that's just the way she is.