That's a common leak point for those transmissions. I have been told you can replace that seal inside the vehicle by removing the valve body but other people have said that's not a good way to do it.
You can go on for a while, until it gets too messy. In my case, since I use it as a daily driver and occasional road trip vehicle, I decided to have it dropped and had it rebuilt all around (rather than waiting for it to fail). It cost a bit, but in those transmissions can last forever if you keep them up.
'89 Wrangler YJ
97 Grand Cherokee (ZJ)
87 MB 560SL roadster