I've been having trouble with Charge rank 2 as well.  I swam my orc hunter up to Westfall to tame a Goretusk, as those are the only boars I know of that have charge 2 and I wanted to teach it to my little black Durotar boar.  It said that I learned Charge 2 from that Goretusk, but when I went to teach it to my old boar, Charge 2 wasn't in my list of known skills.