There have been 3 previous World Series where a team has come back from being down 3-1, winning the final 2 on the road: 1979, 1968, 1958.
Not surprisingly, a common thread running through all of those series is that the winning team allowed very few runs over the final three games.
1979 Pirates (Orioles): 2 R, 27 IP, .67 ERA 1968 Tigers (Cardinals): 5 R, 27 IP, 1.67 ERA 1958 Yankees (Braves): 5 R, 28 IP, 1.61 ERA
Can the Rangers pull this off? Unlikely, but they have the right guy pitching in Game 5. Even though Cliff Lee was knocked around in Game 1, he still has great career numbers in the postseason and it's easy to imagine him shutting down the Giants in Game 5, just like he did with the Rays and Yankees.
C.J. Wilson would start Game 6, and he pitched decently against the Giants in Game 2 (6 IP, 2 ER) despite the 9-0 final score. In Game 7, the scheduled matchup would be Jonathan Sanchez v. Colby Lewis, and it's hard to see how that favors the Giants too much. They would be at home (the home team has won the last 8 Game 7s), but the Rangers would have the momentum, and Sanchez has struggled recently.
Even though the Giants have been hitting well in this series (especially at home), they're not a great offensive team, so it's not out of the question for the Rangers to keep the score low. Unfortunately for the Rangers, they have to face Tim Lincecum and Matt Cain in Games 5 and 6, so they're going to have trouble scoring too.
The Rangers will be lucky to send the series back to San Francisco, but they have the pieces to go deep into the series too, as long as Cliff Lee starts pitching like Cliff Lee again.