It will be faster to just replace the whole unit in the field or side road and then fix the u-joints later to make that the spare if that was the case.
Everything can break! I ran my 86 Toyota pickup in 7 desert races and you would not believe the things that can fail and the carnage I saw from other trucks.
well i better haul another winch around with me just in case cause it might fail:Wow1: theres a point where the weight that u carry starts too make it easier to break parts and a dshaft isnt light by any means. im just not seeing the need for a spare front shaft as it will never get hit and bent. if you do take it out of 4wd and go home. or get somewhere u can make the repair. do you carry spare rear axle shafts? theres a greater chance of braking one of those than a front driveline. just sayin.