As of right now, assuming everyone is healthy, and without knowing the specific matchup, this is the 25 man playoff roster I'd like to see the Rangers go with in the first round:
I think most of this is pretty self-explanatory. If Rich Harden pitches like Rich Harden down the stretch, I'd bump Hunter from the post-season roster to make room for him. You don't have much of a need for a long reliever, so no real need for Scott Feldman...plus, both Harrison and Kirkman can give you multiple innings if a starter gets knocked out early. Having Kirkman in the pen helps you play matchups late in the game.
Willy Taveras is the guy that I'm guessing most people are surprised to see on this last. But in a postseason series, with guys like Guerrero and Molina in the lineup, the Rangers need a quality pinch runner. And given that Julio Borbon is a candidate to be lifted for a pinch hitter late in games, the Rangers probably want to consider having a quality defensive centerfielder available on the bench.
If Gentry were healthy, he'd be the obvious choice. And I could understand maybe taking Taylor Teagarden, Max Ramirez, or even Chris Davis instead, the first two to give you a righty bat and 3rd catcher, the last as a defensive replacement at first base late in games.
But in a playoff series, I think Taveras makes the most sense...I think a pinch runner/defensive replacement is going to have more value than a third catcher, a defensive first baseman, or a second utility infielder.
UPDATE -- Looking at the schedule, there would be four days off between Game 1 and Game 4, and between Game 2 and Game 5, so you wouldn't need a 4th starter in the ALDS.
Given that, I would take Hunter off the roster, and replace him with Taylor Teagarden.