Have a look at http://httpd.apache.org/docs/current/mod/mod_rewrite.html and the examples; specifically you are helped by the fact that:
- REQUEST_URI
The path component of the requested URI, such as "/index.html". This notably excludes the query string which is available as as its own variable named QUERY_STRING.
Which then lets you do things like
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^/foo/(.*)$ http://server2/$1 [P,L]
RewriteRule ^/foo/(.*)$ http://server1/$1 [P,L]
and so on. If it is the entire server - remove /foo/ and the / before $1 - if it is server specific - put an extra RewriteCond in front of it to limit to a specific host and so on.