The only thing worse than a dog with an on-going diarrhea issue is multiple dogs with diarrhea. It gets old real fast.
I too, would seek a second opinion to try to pinpoint the cause.
Dr. Doug gave excellent advice on things to check related to the diabetes issue.
As far as diet, I think you have to find what works for your dog and stick with it. I was never a fan of changing foods. If it ain't broke, don't fix it.
As far as the hamburger versus chicken -- I was doing the hamburger for my old boy and he wasn't tolerating it. Chicken worked for him. Every dog is different - just like people.
OP: How is your girly doing? Since the original post was July 4, I'm assuming you havent' had her back to the vet yet. I would be interested to hear what the solution is when the vet figures out what is going on.