I've never heard of Strong, Swift or Vitale. Grullon is a lefty pitcher out of the Dominican Republic who was getting some run a couple of years ago, and who is still just 21 years old. Hilligoss was acquired from the Yankees for Greg Golson, but is basically a 26 year old organizational depth guy.
The two moves that are, while not necessarily surprising, at least noteworthy, are the release of Kiker and Doyle.
Kiker was the Rangers' first round draft pick in 2006, the guy the Rangers settled for after missing out on Tim Lincecum. He was viewed as a bit of an overdraft at #12, and the Rangers passed on Texas native Kyle Drabek (who was viewed as very talented, but with off-field issues) among others (including 2006 first rounders Daniel Bard, Travis Snider, Joba Chamberlain, Jeremy Jeffress, and Ian Kennedy) to nab Kiker.
Still, Kiker appeared to be justifying the Rangers' picking him for a while, pitching his entire age 21 year at Frisco in 2009 while putting up solid numbers.
Even in 2009, though, there was a warning sign, as Kiker's walk rate elevated to 4.7 per 9, and he hit 17 batters in just 126 innings. That was a precursor of things to come, as Kiker's control completely abandoned him in 2010 and 2011, as he walked over 10 hitters per 9 each season, and hitting 15 batters in 84 innings over the two seasons.
With a fastball that wasn't hitting 90 and zero control, it seemed inevitable the Rangers would end up cutting ties with their 2006 first rounder.
Doyle, on the other hand, was a 4th rounder out of the University of Oklahoma in the 2009 draft. He was getting some buzz in the 2009-10 offseason as a guy who could move quickly and contribute out of the bullpen in the near future, but he was ineffective in A ball in both 2010 and 2011, and apparently has been passed in the pecking order by enough other pitchers that the Rangers felt it made more sense to cut the 24 year old loose.