I'd definitely try the Greek yogurt. And maybe try a mix of creme fraiche and the yogurt, if the 2% doesn't work out. Just be aware of the high amount of bad fats, and compensate with the rest of your day (for instance, if you usually have some 2% cheese during the day, don't eat any on the day when you make this dish) so you don't go over your usual amount of saturated fat.
We have a really yummy spaghetti squash gratin recipe that uses creme fraiche. I make it maybe once every 2-3 years, so I figure it's okay.