== SOLVED! ==
As it turns out, the reason ruid2 was working on some websites and not others was permissions - some of the directories were owned by apache, which apparently confuses ruid2. Once I ran chown -R username
saserv to reset all website permissions all was well. So... if you are wondering how to get ruid2 running on a system similar to mine, the instructions are below 
My system is as follows:
CPU:
GenuineIntel, Intel(R) Xeon(R) CPU E31220 @ 3.10GHz
Operating System:
CentOS release 6.2 (Final) (Linux 2.6.32-220.7.1.el6.x86_64)
Plesk Version:
Parallels Plesk Panel v10.4.4_build1013111102.18 os_CentOS 6
Apache Version:
Apache/2.2.15 (Unix)
ruid2 package installed:
mod_ruid2-0.9.6-3.el6.art.x86_64
Now to Install
(1) As 'root' cd to /root
(2) Download the latest atomic-release rpm from
http://www6.atomicorp.com/channels/a...5/x86_64/RPMS/
(3) Install atomic-release rpm: # rpm -Uvh atomic-release*rpm
(4) Install mod_ruid2 rpm package: # yum install mod_ruid2
This will result in these files on your OS:
(1) /etc/httpd/conf.d/ruid2.conf
(2) /usr/lib64/httpd/modules/mod_ruid2.so
(3) /usr/share/doc/mod_ruid2-0.9.1/
(4) /usr/share/doc/mod_ruid2-0.9.1/LICENSE
(5) /usr/share/doc/mod_ruid2-0.9.1/README
After you instal mod_ruid2-0.9.6-3.el6.art.x86_64 (or current version) run the command
apachectl -t -D DUMP_MODULES
Look for something like this to result:
Loaded Modules:
core_module (static)
mpm_prefork_module (static)
http_module (static)
so_module (static)
auth_basic_module (shared)
auth_digest_module (shared)
authn_file_module (shared)
authn_alias_module (shared)
authn_anon_module (shared)
authn_dbm_module (shared)
authn_default_module (shared)
authz_host_module (shared)
authz_user_module (shared)
authz_owner_module (shared)
authz_groupfile_module (shared)
authz_dbm_module (shared)
authz_default_module (shared)
ldap_module (shared)
authnz_ldap_module (shared)
include_module (shared)
log_config_module (shared)
logio_module (shared)
env_module (shared)
ext_filter_module (shared)
mime_magic_module (shared)
expires_module (shared)
deflate_module (shared)
headers_module (shared)
usertrack_module (shared)
setenvif_module (shared)
mime_module (shared)
dav_module (shared)
status_module (shared)
autoindex_module (shared)
info_module (shared)
dav_fs_module (shared)
vhost_alias_module (shared)
negotiation_module (shared)
dir_module (shared)
actions_module (shared)
speling_module (shared)
userdir_module (shared)
alias_module (shared)
substitute_module (shared)
rewrite_module (shared)
proxy_module (shared)
proxy_balancer_module (shared)
proxy_ftp_module (shared)
proxy_http_module (shared)
proxy_ajp_module (shared)
proxy_connect_module (shared)
cache_module (shared)
suexec_module (shared)
disk_cache_module (shared)
cgi_module (shared)
version_module (shared)
fcgid_module (shared)
perl_module (shared)
php5_module (shared)
python_module (shared)
ruid2_module (shared)
ssl_module (shared)
If it's there, you're good to go
Next, make the required additions to the ruid2.conf file.
Note that since Plesk set my master account up using the /var/www/vhosts/mydomain.com/httpdocs path, I added this first:
<Directory /var/www/vhosts/mydomain.com/httpdocs>
RMode stat
RUidGid apache apache
RGroups apache psaserv
</Directory>
Then, it set up additional domain websites using the /var/www/vhosts/mydomain.com/myotherdomain.com path, so I added this for each of those:
<Directory /var/www/vhosts/mydomain.com/myotherdomain.com>
RMode stat
RUidGid apache apache
RGroups apache psaserv
</Directory>
Hope this helps someone. (Definitely wish I had found something similar.)
Good luck!
