At 204K on a transmission, you can be almost sure that something is wrong. Especially if the previous owner "serviced" it regularly. See, the transmission fluid, after a while, has some clutch material in it, and it actually allows the clutches to continue to operate smoothly. Once you change out the fluid = slipping tranny. I've seen it too many times to count.
My suggestion, put another transmission in it, or have the one in their rebuilt. You're going through the trouble of pulling the engine, might as well.