Hey this is kind of a long shot.....but I remember a TSB for the 98s at least that meantioned that the trannie breather hose can get clogged causing some fluid leakage. I'm not sure if that would apply to yours or not being a 98.5 but since they haven't been able to find another source of the leak it might be worth looking into.
Good luck!
Oh yeah.....and to everyone on here......always save all receipts anytime you have anything done to your vehicle....I mean anything. Even little things like oil changes....I know several people who had to learn this the hard way.
