tranny to dana18 oil exchange ?

Iv got a t14 tranny and a spicer 18 t case. It seems like the oil from the tranny is flowing into the t case. Causing a low oil level in the tranny and overfilling the t case which causes it to leak out of the spedo hole and other places. I cant remember if i put the big coned washer /spash shield behind the transfer case input gear. Would that cause the problem? Seems to me that that could be the only thing that would cause this