I have done all kinds of research and tried many different things. I know this question has been answered many times, but none of the suggested solutions are working for me.
Just ran into this myself (OS X Lion 10.7.5). My exact message was: Nokogiri was built against LibXML version 2.8.0, but has dynamically loaded 2.7.3
I tried a few suggestions mentioned here, none worked, but this did:
gem install nokogiri -- --with-xml2-dir=/usr --with-xslt-dir=/opt/local --with-iconv-dir=/opt/local
The explanation is: "This happens because the Lion system default libxml2 (loaded at bootstrap) is used, regardless of which libxml2 Nokogiri was built against."
Credits to: https://coderwall.com/p/o5ewia