What's causing the crack to grow is called fatigue. Fatigue is a cyclic event, whereby each cycle causes the crack to get a bit bigger. Your car going over bumps during driving is the "cyclic" part of it.
I'm not sure if welding will help. Those parts are heat-treated to be resistant to cracking and failure. Obviously something is wrong with THAT part.
I would continue to try and find a replacement and to check that part out for any sign of fatigue or cracking before it's installed.
And it only took 5 years of engineering school to learn that...
