I had an oil leak there too on my Z. Assumed the same thing, that the O-ring had a problem and ordered a new one.Couldn't get the unit electrically unplugged so removed it anyway. Jack it up high in front or drain the lube. After cleaning it all up and installing the new O-ring in the grove I noticed the O-ring didn't stick out of the grove at the bottom but did at the top. Micing the grove confirmed it was cut deeper on the bottom then the top indicating it wasn't centered when machined. Of course it had to be on the bottom where the oil was. I noticed the actuator housing and the diff case actually made solid contact so I cleaned all surfaces with braklean to get all film off then use some high temp RTV on the noise for lube and a ring where it makes contact, slid it in, installed the bolts evenly, let it sit over night then let it off the jacks and added any extra lube it may have needed. Yes I did also install the useless O-ring. No leaks to date and it's been several years now. Be sure to have it in 2wd and the slide all the way to the right when installing the actuator.
You should also know that some have reported that oil had got past the gear drive shaft seal into the gear drive section and motor. From there it can come out where the wires enter so look close as to where the oil is coming from. Good luck...take your time and go easy. Here's a pic of what you will see:
