rear driveshaft/ Differential yoke

I have a two piece driveshaft for my 1/2 72 gmc longbed. The nut that attaches the rear yoke on the driveshaft to the differential has come loose and almost fallen off three different times. Its wearing down my gears. I'm about to put a lock washer on it to keep it from comming loose until I can have it looked at. Any ideas why this is happening and what can I do to keep it from doing it again? Thanks for the help.!