You have too much room for that shaft to slide back and forth between the axle and where it hooks up to the differential. Take the shaft off. Cut 2 slices of fuel line about 1/8" thick. You're going to use the slices of fuel line as a spacer on the end if the shaft where it fits into the axle and the output joint of the differential. Put it back together and adjust the turn buckle to reduce the amount of end play of the shaft sliding back and forth. You can also paint your axle shafts with a bright color like white or yellow so you can find them if they come out but this should stop that from happening.