Shane had a gun to Rick's face and there was no talking the dude down. It was pretty obvious that he sought to take Rick out of the equation long ago and just couldn't find the right moment to follow up (hell, he almost did it in season 1 when the two were out scouting). What was Rick suppose to do? He has a pregnant wife and child. Also, releasing that type of news to a group of individuals who are already on edge following the Sofia incident (not to mention the fact that he was unsure as to whether the information could be trusted, based on who he received it from) just seems like a formula for disaster. Why cause unnecessary worry?
He's fed up, not crazy.