Have you tried:
ProxyPassMatch ^/(.*) http://granny-server:8181/$1
ProxyPassReverse / http://granny-server:8181
Or even more simply:
ProxyPass / http://granny-server:8181/
ProxyPassReverse / http://granny-server:8181/
I wrote up how I deal with Apache reverse proxying &and Tomcat over here if you want to compare/contrast what you setup with what I use.
You might want to add this to add a trailing slash to the URL:
# Settings for adding a trailing slash to the URL
RewriteEngine On
RewriteCond %{REQUEST_URI} ^/$
RewriteRule ^(.*)$ http://%{HTTP_HOST}$1/ [R=301,L]