This took me enough time to get right to post a little blog post about it:
RewriteEngine on
RewriteMap lc int:tolower
RewriteCond %{REQUEST_URI} (shared|Shared|download|Download)/[A-Z]
RewriteRule (.*) ${lc:$1} [R=301,L]
Note that this doesn't work in .htaccess - you have to put it higher up in the Apache configuration.
I tried extra RewriteConds, to no avail, somehow Apache seems to add [OR]s anyway, even though the documentation states that you have to specify them (there is no [AND] directive). I tried adding the "shared" and "download" to the RewriteRule, leading to infinite redirects. Of course adding [NC] to the RewriteCond will also lead to infinite redirects.
Apache configuration is so much fun.