So, the Rangers can't select a player in the Rule 5 draft, because their roster is full at 40 right now.
However, they can work out a deal where a team selects a player in the Rule 5 draft and then trades that player to the Rangers. That's how the Rangers ended up with Fabio Castro...the Royals selected him in the Rule 5 draft, then traded him to the Rangers for Esteban German.
So it is possible that the Rangers could end up as players in the Rule 5 draft in a couple of weeks, despite having a full roster.
And the guy who seems like someone the Rangers might be targeting is Cubs LHP Donald Veal. Last spring, when the Rangers were talking to the Cubs about a Marlon Byrd trade, Evan Grant said the Rangers wanted Veal, Jose Ceda (since dealt to the Marlins for Kevin Gregg) or Sean Marshall for Byrd. So the Rangers have, one would assume, scouted him pretty hard and are probably still interested in him
Veal throws hard, supposedly has a couple of good secondary pitches, but has lousy command. He struggled this year and hasn't done that well in the AFL. But the Rangers have made it clear that lefty relief help is a priority, and Veal was impressive against lefties in the minors last year, not allowing a homer to a lefty batter and striking out almost 9 per 9 IP (although walking almost 7 per 9).
The thing that mitigates against taking a Rule 5 guy is that the Rangers already have Dustin Nippert and Scott Feldman who are out of options, and taking someone like Veal would reduce the team's flexibility. But I have to think that they are taking a close look at Veal, and a few other power lefties like Pedro Figueroa of the A's organization, to see if their second lefty out of the pen might not be someone they could pluck from another team.