I would suggest a call to S&W, it's possible they may be able to fit a new yoke. However, since I have never heard of a yoke cracking I would suggest that you look it over very carefully to make sure that what you are seeing is acually a crack.
I'd also like to see some pics because this part is under almost zero stress so it's a bit hard to believe it cracked. The only explanation I can think of is that it wasn't properly machined when the stud or yoke tube was installed. If that's the case it sort of puts the late "pinned and recessed" era into the "problem prone" category.