stripped driveshaft?

on what was to be the last striper trip of the year last season (dec), about 5 minutes from home, the engine jumped up rpm's and the prop stopped spinning. could no longer get outdrive into gear forward or reverse (no discernable click per usual when throttle is pushed). also, no bubbling out the prop, i.e. no water being circulated thru the engine, presumably due to shafts not spinning the impeller. started engine up today for first time since, runs fine, and next week i am going to pull the outdrive and see what's up, but am i probably looking at a stripped driveshaft where it couples with the engine? there is a whirring noise (like spinning without gears meshing) that gets louder with the rev of the engine, seems to be coming from right at the transom, on the outdrive side. any ideas on what i should look for when i take it off? how easy is it to put a new driveshaft/ujoint assembly in? any input appreciated.

oh yeah, and there were no signs of anything wrong up until the moment she wouldn't spin the prop anymore, and the prop does not have any damage, so i didn't hit anything.

oh yeah, '79 470, w/model 1 outdrive.