Yes if you overfill it it will definitely leak from the original front input shaft seal. Remove the fill plug from the transfer case and the transmission and let them both drain until they stop. Then put the plugs back in. Overfilling is a bad idea. Regardless, they usually leak somewhere, until their empty! Notoriously these old transmissions/engines and transfer cases find a way to leak.
https://willysjeepforum.kaiserwillys...se-Lube-Levels
What lube did you use to fill it up? If you have a good GL4 for synchronized transmissions in there, there could be a few things on the second gear shift. The most probable is a worn 2nd gear synchronizer.