2wd - 4wd

ok, my cousin has an 86' chevy C20 custom deluxe, 350, with a 3-speed(with granny gear)

Well, he loves off-roadin in this beast, but he is sick of his 2wd, is there a way to convert it to 4wd, and what all is involved??

thanks for your time, and please, no one be a dick bout this, it is just curiousity....