Thanks for the information you provided...
I believe the shift in rankings will be primarily due to a weak link profile for your domain but also a stronger backlink profile from some of your competitors which has seen them rise above you in the organic rankings. These changes coming since the additional Penguin algorithm updates rolled out by Google. (Incidentally, have you seen a decline in Yahoo/Bing if you monitor positions at these search engines too)?
When seeking link opportunities, it's not advisable to chase PR - if 95% of your backlinks all come from pages with a PR3+, then this in itself looks unnatural and will have been a signal flagged.
It is also possible, that those domains you have previously sourced backlinks from via building content relationships could also have suffered as a result of the latest algorithm updates - if they look to sell/provide backlinks using their page PR as selling point, and subsequently link out to various websites from sidebar placements (which they do looking at some of the pages containing your backlinks), this could also have made your domain suffer as less weight/authority is being passed through your links.
I would recommend acquiring some new backlinks, don't worry about the PR of a page and don't focus on using keywords in your anchor text. Perhaps consider some guest content at a few blogs, writing a few pieces of content at each related to your products/services using an author byline with branded backlinkS - for example:-
<name>is the marketing manager at RentalInsurance.org for a leading provider of renters insurance in New York.</name>
Perhaps also consider contributing via comments to some blogs, doesn't matter whether backlinks are nofollow or otherwise, just ensure your brand and where possible,link back to your site (using your name for commenting instead of keywords) via making some relevant and genuine comments to other blogs.
Build a few handfuls of backlinks such as these which will counterweight the links in your existing backlink profile and ensure there are some more genuine and natural links being built.
I would expect this to prove a step in the right direction however there is also the possibility Google could also be looking at onsite elements and the practically exact match domain name too which have also seen changes from recent algorithm updates.
Hope that helps for now.