We've never used whole milk, my boys nursed beyond a year nad we slowly intro'd 2% milk into thier diets around 18 months, and around 2 we went to 1%milk, but they still only drink jsut a cup a day at dinnertime. They both eat plenty of greens so they get thier calcium.
As far as the constipation.... how often is she pooping? My oldest (5 1/2) poops every day at almost hte same time every day LOL! He's like clockwork, always has been. My youngest really varies, he goes 3-4 days without pooping, then poops 3-4 days in a row. He sometiems seems to strain when pooping (he's trained, has been since 15 months old) but its never constipated poop.
she is truly constiapted hwen her poops are dry, hard little pellets. If she isn't pooping dry hard pellets, then she's not relaly constipated, just straining to poop, which is normal. Pooping every day isn't really 'required', and kids vary so much. As long as her poops aren't hard and dry, ten she is fine, so dont worry. :)