What connection is there if any between Biff's return and Willy's inability to get past Yonkers?
There is indeed a connection. It's one of those details us literature buffs love to find in critical analysis. In this case, Willy is physically unable to drive, as he is aging, losing his vision and motor skills. But there is a mental barrier as well. It is here that Biff caught him committing adultery so many years ago, & Willy cannot face that memory. Because Biff is returning home as well, Willy chooses to live in his dreams, rather than facing reality. He picks & chooses which memories to emphasize & which to forget, & Biff once again becomes a living symbol (to him) of what can be achieved. Willy's desire for Biff's success is so strong that it pulls him home again, to attempt a reconnection. Unfortunately, it is too late for that, and Willy's inability to continue as a traveling salesman, along with Biff's return home, becomes the catalyst for the unraveling of their lives.