Proulx uses the wreck of the Thresher to give a historical reference and establish the time. It's clever thinking on her part because it's a notable news item that guys like this might talk about. It also happens in April, just before the summer the pair spend up at Brokeback Mountain.
The Thresher was the first nuclear-powered submarine in the U.S. Navy. It was commissioned in 1961 and spent two years undergoing trials to test all of its new-fangled bells and whistles. One the 10th of April, it began deep diving tests off the coast of Massachusetts, accompanied by a rescue ship in case things went wrong. They did. The rescue ship got some garbled communications suggesting big problems, then nothing. The sub went down and took 129 crewmen with it. Check out the Navy's page on it for more details.
Pop Culture References
The Strawberry Roan
Ennis and Jack weren't exactly Top 40 kinds of guys, but they knew the lyrics to "The Strawberry Roan," a country and western song written for a movie of the same name. The film was released in 1948 and starred legendary singing cowboy Gene Autry. It's about a boy thrown by a horse, the strawberry roan, and left paralyzed. Autry's character trains the horse to be gentle, returns it to the boy, and maybe helps him forget the whole "debilitating injury" thing. Autry also sings the title song, which Ennis apparently knows. It's also about tragedy and learning to bear injuries (physical and otherwise), which Ennis is clearly down with. The song makes a clever way of getting into Ennis's head a little bit without disrupting the distant spirit of the book.