I couldn't figure out where the crack was either. If Bob has spotted it, yes it can be welded and yes it must come out of the car and the carrier removed. You'd cook the
oil and start a fire trying to weld it in place. Also the area to be welded needs to be thoroughly cleaned inside and out.
After welding, it should be stress relieved. I'd also look for other indications of fatigue in the housing while it is out. The crack you have is stress induced.
Could even be that replacing the housing would be a good idea. That wouldn't cost all that much. There are lots of 9" Ford housings around. I'd call that approach cheap life insurance.
Al