The Rangers were, at various points, 6-0, 9-1 and 14-7 before starting their decline which has resulted in them being 18-18 as of today. They were 16-11 in April and are 2-7 in May. They also lost 4 of their final 6 games in April. What happened?
They've fallen 12 slots in each category, but it's the pitching that is most worrisome because they're not going to the playoffs if their team ERA is one of the worst in the league. The offense is likely to bounce back because some of their best hitters (Ian Kinsler, Adrian Beltre) are mired in slumps that they'll probably be out of soon. Offense will still be an issue if Josh Hamilton and Nelson Cruz don't come back and produce, but it should improve.
Some of their pitchers have been good in May (Alexi Ogando, C.J. Wilson, Dave Bush) and some have been dreadful (Cody Eppley, Ryan Tucker, Pedro Strop, Arthur Rhodes). These four relievers have combined to for a 13.09 ERA over 12 appearances and 9 IP.
The rest of the staff has a respectable ERA of 3.63. Eppley made his major league debut on April 23rd and he allowed all of his runs in May in one disastrous outing against the Yankees (6 ER, .1 IP). Tucker has been allowed runs consistently and he probably needs to improve quickly.
So, it might not be as bad as it looks, although they're a different team without Hamilton and Cruz. They're fortunate to be in what might be a relatively weak division (although watch out for the Angels if they can get their rotation going behind Haren and Weaver).
Their offense is still good (somewhere between 3rd and 15th without Hamilton and Cruz), and their pitching is serviceable, although they need to figure out the bullpen quickly. It would help if they had another phenom come up from the minors like they did last year with Alexi Ogando, who debuted on June 15th and had an ERA of 1.30 over 42 IP the rest of the way.