Cracked chainstay weld -- now what?
Get in touch with Surly even if it out of warranty. The worst they can do is say no, and even if they won't fix it, they should be able to offer advise on the the best way to proceed. They live or die based on peoples perception of the brand, so have a vested interest in helping you resolve this kind of problem.  

Any competent engineering shop should be able to make a reliable repair on Cromo. frames.