Leanne was shot by a cop.
After Leanne drove away with Bill's truck, Hank tells Bill to call the police, but Bill says that only if she comes back. Hank decides to do it himself later on that day and phones the police about Bill's truck. The police track Leanne down and pull her over. Leanne, instead of cooperating, gets aggressive with the cop, nearly stabbing him in the neck with a fork. The cop pulls a gun out and shoots Leanne out of self defense. Leanne is now dead, which explains why she makes no further appearances in the series. Peggy (and the rest of the neighbors) couldn't be happier.
