ORIGINAL: traxxaspede
That was a hilarious yet very informative video. I think that should clear things up for you. If your wondering about limited slip, or locked differentials, it's because if one wheel is in mud (or something else) and can't turn the other wheel will just spin, like a 1 wheel drive car. locking the differential, or puting in a spool will make that problem go away.
You should only lock the diff if you are going 70+mph doing high speed runs on asphalt. Locking the diff for anything else is just plain stupid.
You have a ball diff right? If you have a fast motor, you want to get the diff screw tight. You can check this by puttings the car on the ground and hold the chassis down. The diff will make a barking sound if its too loose. If it is correctly adjusted, the slipper will slip a little and the car will try to go foward.