I've had a similar task. Problem is, most software configurations just
do a simple round robin, no balancing. My solution to balance ldap
between six edirectory servers is to use a Kemp Loadmaster 2200
Appliance. The box (we use two of them in HA) is cheap, easy to manage,
flexible and very reliable. Balances nearly whatever you need.
You are able to set rules how to balance (weighted round robin/weighted
least connections and so on)