hey im wondering the same thing..
apparently u can do it, just pull the fuse out. and on the the dash wen u turn it on, the 4wd lite shouldnt show so that means ur running 2wd.
ive got a n/a r34 gt-four and yes they use GTR gearboxes. had mine converted to manual.
altho i dont want my car to b 2wd coz then ill b pressured into doin burnouts lol. tyres dont come cheap specially wen u have 265/35 19" wheels