Umm, both wheels have power on an open diff. It's not 1-wheel drive. As long as you aren't spinning tires, there is absolutely no difference between an open diff and limited slip. If one wheel starts spinning and the other doesn't, an open diff will route extra power to the wheel that's spinning, and if you keep romping on the throttle you'll end up with the spinning tire having 99% of the power and the non-spinning tire having 1% of the power and you'll go nowhere. However, if both tires break loose at the same time and continue to have similar traction, you'll get two black marks (or two ruts if you're on sand).
And yes, force has a lot to do with it. The traction on the tire is determined by 3 things; how sticky the ground is, how sticky the tire is and how much force is smashing the two together. Less force equals less traction; hence why the inside tires spin first (under throttle) or lockup first (under braking). You can also use this to get both tires spinning on a car with open diff by shaking the wheel back and forth (well, it works on a couple front-wheel drive Hondas; not sure about RWD).
Edit: Ok, here's the easiest way to tell if a car has LSD: put the right side in sand and the left side on pavement (or vice versa) and drop the clutch. LSD should accelerate the car with the wheel on pavement while spinning the tire in the sand. Open diff should just spin the tire in the sand and go nowhere. In a car with really strong LSD or a solid axle/welded diff, the car would start moving and the tire in the sand wouldn't spin because it's moving at the same rate as the tire on the pavement (it would turn, just not spin and fling lots of sand everywhere).
Edit2: In the above test, make sure you have another car and a chain or rope, in case you don't have LSD and get stuck.
