Moon Lake to Rock Lake is pretty obvious when you get up there. I did see a couple of cairns, but, I don't know where there would be a "trial" and its totally unecessary. Its pretty easy terrain to hike (I ran it and it would be easy with a pack to hike), you just have to make sure you aim for the right "pass" to go over between the lakes. Its not a lot of elevation gain to it. Don't sweat it, you'll easily figure it out when you get up there and you'll see why you don't need to worry about a trail....its just wide open, high alpine terrain.
And yes, the road to the Pine Creek trailhead can easily be travelled on by any vehicle. Its pretty well maintained. Beautiful area up around those lakes (Emerald Lake is amazing) and not many people up there (I've never seen anyone).