Rake 'no such file to load' but gem exists and loads

I've been working on a production app for about 3 years, and after a
recent upgrade began receiving the error 'no such file to load --
<FILE>' when running rake. The files are always gems that it is trying
to load during initialization.

The strange part is that I can load the app using ruby script/console
(or server), and I can open a shell, load the gems just fine by
requiring rubygems and then the names of the gems -- so I know they are
installed fine (and were previously working).

I've uninstalled and reinstalled rails and rubygems... to no avail. I
have a spare development machine in which the app works/develops fine,
it's just my primary that fails.

Help !??

Russell