If you move your axle forward 1.5" you will more than likely need to shorten the driveline. Otherwise it will most likely bottom out. If you move the axle back you may need to lengthen it. You only have so much slip available. Too long of a driveline will bottom out and too short of a driveline you risk it pulling apart. I generally always tried to make standard toyota rear shafts 1.5" from bottoming out at ride height.
