Transmission fluid leaking back, 1988 Ford Bronco

your output seal on the tranny or input seal on your transfer case is bad. you have to pull your transfer case to tell. this is a pretty straight forward job. block wheels. put transfer case in neutral. drop front and rear drive lines.diconnect transfer case shift linkage. there is a short commponent connecting the trans to the transfer case. the bolt on either end face each other. remove the bolts to the transmission. the transfer case is heavier than it looks, i suggest getting help. pull back on the transfer case until it will drop down. it will pull out a bit because of the trans output shaft. replace what will probibly turn out to be the tranny output shaft seal and reassemble in reverse order.