How to replace Leatherback Baloth in the post-Zendikar environment in a deck of this type?
Well, assuming you're happy with how the deck was working up to now, the most important thing is to preserve the mana curve.  That is made extra important of course by the use of birthing pod.  Once you constrain to 3-cost green-only creatures, the list of options in standard is quite manageable.  Especially considering you can exclude the infect creatures.

http://magiccards.info/query?q=t%3A%22creature%22+%28c%3Ac+or+c%21g%29+cmc%3D3+f%3Astandard&amp;s=cname&amp;v=card&amp;p=1

I would say a fourth Dungrove Elder is an obvious recommendation.  I also like Sylvok Replica a lot for this deck.  The option to use him as an artifact removal is great, and he still finds Thrun for you if your opponent has no artifacts.