I had a model 37 that developed the frame crack last year. Called the S&W service department and they emailed me a prepaid shipping label with shipping instructions. Fed EX picked up the package at my house. You must be there to sign off on the package. A couple of weeks later S&W called and informed me that the gun was not repairable and that they would send me a new 637 as a replacement if I so desired. I picked it up at my FFL a few weeks later. The only cost to me was the transfer fees at my FFL. Seeing as how I was not the original owner of the model 37 and that it was made a year or two before S&W started their lifetime guarantee program, I was very happy with how S&W resolved the issue. There are several posts on this forum about the aluminum J frame crack issues.
John