Well most are made so that they COULD be raced. AE,Losi,Mugen,XTM,HPI(Rush), Traxxas all offer 2WD b/c that is what is race legal in that class. HPI,MRC, GS and others make a 4wd truck, but they are the exception to the rule. If you want to race them, you will be out of luck at most tracks.
4WD is great for bashing, and it makes it easier to get around the track with. 2WD ST racing is one of the harder to master classes. With all the power that todays .12-.15's and even .18s offer, it takes a gentel finger to get the truck around the track at a good clip. But when your bashing, who cares? Peg the gas and let er rip.
2wd vs 4wd is a good debate for bashing.