I use Akregator. It supports archiving (including ‘keep everything/unlimited duration’), has search, can be configured to fetch either full article or RSS snippet on read, and does support ‘share’ (at least on identi.ca and twitter, I don’t use that so haven’t bothered to look for plugins/extensions for other services. Obviously cut/paste of the URL is always an option in a pinch).
As for past feed items, I think that you may be out of luck. Most RSS-autogenerator widgets only provide the last X items, and either ‘age out’ at some defined period, or delete oldest first at some defined number of articles. No amount of poking at the feed reader is going to get it to fetch articles that simply aren’t defined in the RSS/Atom file anymore. (One trick, if you really care about a given feed, is to run the feed URL through the Internet Archive Wayback Machine, that will give you at least snapshots of certain parts of the past; but it is a bit of a pain).
The fact that Akregator is a KDE application, and thus mostly for Linux/BSD, with Windows and OSX much less mature, if they’ve gotten them up and running at all, may be a problem for you, depends.
