No you have to do it for any car/truck. It's easier to rebuild an engine then to change drives. It would be faster to take to cars body off the frame on put it on a rear wheel drive car
(Just joking don't do it). I've done 2x4 to 4x4 with my dad as a kid and it's no small job. Don't know why you would want to convert your car to rear wheel drive but my guess is that it won't be worth it.
Rasb