TH 400 leak

[zac]

Last fall I bought a rebuilt TH400 and an np203 off craigslist. The guy had everything bolted together already, including the crossmember. We just loaded it as a unit with an engine hoist. He said it was "ready to go." I unloaded it with my hoist, set it on some jacks, kicked it under my '79 K20 and bolted it in. Everything works fine, but after using it for a few months now it seems to leak fluid between the trans case and the t-case adapter. I'm starting an axle swap this week, so I might as well fix this too. What seal or gasket is leaking, or what did this guy possibly forget or screw up? I'd like to know what to look for before I get into it. Thanks.