Is it safe to drive temporarily with a cotter pin missing on the lower ball joint? Old cotter pin broke while replacing axle.

 

I would suggest you use something else. I've seen instead of a cotter pin: Metal Hanger Wire. A piece of 10 gauge solid wire or even a nail and then bend it to stay secure. These are all just temporary fixes until you obtain the correct cotter pin. You might even consider using half of the old cotter pin and just bend it to fit in. Just don't forget to get a new cotter pin. There really is no pressure on the cotter pin, it is there just to prevent the nut from unscrewing.