if you take out the spider gear then your car wont move lol. its hard to explain without a picture and seeing as im in class i have no way of getting one however i will attempt to explain it lol. basically inside your differencial there is a gear, that then has 3 pinion like gears attached to it. the other gear is where the power comes from and is connected to the drive gears. the inner spider gears are what drive the tires but since there are independently attacked to the drive gear, the tires can spin at differnt speeds.

i just found the link to it so here it is. ecplains how to do it step by step.

[link]http://www.rccrawler.com/forum/showthread.php?t=12372&highlight=locking+clod+diff[/link]