is it important for drifting ? yes and no some say yes some say no try with and with out see what you like

will it damage the car? yes it can if you dont look after it biggest example is in rc crawlers when both wheels are trying to grab and pull but the crawler doesnt move you can snap a dog bone but i duno if you will experience the same kinda stresses in a drifter

how do you do it?
depends if you have a ball diff or a traditional diff
in a normal traditional
simply put some thing in between the little gears to stop them from moving
eg blue tak, some putty, hot glue
this may be a little vaige best bet is to head on over to www.rccrawler.com and have look at some of the topics over there about locking diffs try in the small scale section in the vmg to crawler tread
basically all you have to do is jam the little gears inside together but still allow the one that is connected to the drive hards to explain with out picies