What's involved in replacing rear crossmembers?

[Infinitrium]

I took my car in for a safety inspection, they wouldn't do it because they said the rear crossmembers are rusted bad. They went on to say if I hit a big enough bump on the road that the rear end would just fall apart. Seems a bit exaggerated but what do I know. Is it worth trying to repair the rear crossmembers? What would be involved in doing it?