Evan Grant has a blog post up about the Rangers' 40 man roster situation, and who could be added and dropped.
John Mayberry, Jr., and Jose Vallejo seem like the slam-dunks. Evan and Jamey Newberg both seem to think Pedro Strop could be added, but it seems to me that if they were going to do that, they would have either claimed Strop in the first place when the Rockies waived him in September, or else given him a major league deal when they signed him this offseason. The Rangers could have moved someone to the 60 day d.l. to make room for Strop in September, even if the roster was full then, I believe.
Omar Poveda is a coin flip, and I don't see Michael Schlact or John Bannister being added. If it is JMJ, Vallejo and Poveda, that leaves the Rangers at 40, although it means that there are probably also guys who will be dropped from the roster later this offseason, such as Wes Littleton and Kam Loe, who Grant discusses as well.