hairline crack rear end housing

Drill some 1/8" holes, one at each end of the crack, then weld it all up. The holes will end the crack.

Was going to be my suggestion too. I usually drill the hole slightly past where you can visually see the end of the crack, as the crack probably becomes invisible to the naked eye, before it actually ends. Drill your two holes, grind out the crack slightly, weld it weld, then grind smooth, and finish it off with a scotch brite pad on a angle die grinder. Shouldnt have any more issues after that.