Grunts, like most sounds/tones, are something that vary by dog. My dog grunts when I've just ended a belly rub and he wants more snuggles or if I'm not paying him attention when he really wants or needs me. I've seen other dogs grunt as a sign to another dog to pay attention to them or to stay away, depending on the tone. If it's a heavy grunt in a dog with a flat nose (a pug, for instance) it could be a breathing issue. You need to pay close attention to the dog and how they're communicating. One answer won't cover this for all dogs.

I've seen dogs scoot for a few reasons, but the primary one is needing their anal glands cleaned out. While it is possible to do it yourself and some of my friends do, I suggest paying someone because a) ew and b) you don't want your dog to associate you with that particular feeling. See the vet!

To be honest, I've never thought to ask ahead and I've never had a problem, but my dog is small and well behaved. I would always just ask, as the other commenters have suggested. Most Uber drivers are pretty cool and happy to work with you.