The biggest cause of front axle shaft breaks is caused by bearing caps that come free. This is caused by the yokes wollowing out and the c-clips popping off as the cap spins in them. Once off, the trunion will tear through the yoke and often times, takes out the ball joints too. If you really let your u-joints and shafts get to this point, spare joints won't be of much help to you. The best thing you can do to prevent this is to upgrade to chromoly shafts with full circle clips AND, maintain your Jeep.