Transmission Fluid Mixing with Engine Oil

[koegh]

Looking for Idea's on the problem.

I have a 1989 K2500 Chevy 4x4 350 Auto with 260k mi. I checked my oil and it was way overfilled and then noticed my transmission fluid was very low. So, I drained the oil and refilled and topped off transmission fluid. Next day the oil was way overfilled and the tranmission fluid low again. How can my transmission fluid be leaking into my engine oil system? I am stumped and looking to see if anyone has any ideas or have heard of this problem.