leaking transmission fluid - 1999 Trooper

I'm trying to get parts info and correct terminology for descibing my problem. I've developed a fluid leak from the small circular area above the fluid pan on the passenger-side of my automatic transmission. I've been told this is 'servo' plate cover composed of a molded rubber gasket. Is the gasket itself replaceable? Is there more to it than that? Someone else told me there is an O-ring here that needs replacing? I've tried finding a detailed parts diagram for this location and have been unsuccessful. Anybody know what I'm dealing with here? What replacement part / parts do I need to consider ? How might I describe this condition accurately to a mechanic? This is not something I would fix myself, I'm just trying to gather enough info to avoid being taken to the cleaners on repair costs. Please help. ( 1999 Isuzu trooper automatic transmission)