I have a similar leak, but mine appears to be coming from the front of the rear end (under the floor).. I have taken the seat off numerous times trying to find it, but no luck so far.. Really need to take the floor off and see if i can see anything..
Mine happens only after it has been run for quite a while.. I ran it for an hour 1 day and it never leaked.. BUT if i mow for an hour it will be dripping after i finish - but doesnt leak when not running..
Are you sure it's not just some hot grease dripping off of the mid PTO shaft or off of the mower deck?? Just my 2 cents.....