There's an oil supply in the rear of the trans to lube the output shaft bushing and speedo gear with a drainback hole to return the oil to the pan. That oil is free to run into the adaptor on uphill climbs. Once the adaptor accumulates some fluid, it can run into the t-case around the input shaft on steep climbs. If I were a machinist, I'd cut a ring to fit in the rear of the trans with a seal to go around the trans output shaft. Not being a machinist, I plan on cutting a flat sheet metal disc to sit in the O'ring seal recess (where the square o-ring sits between the trans and the adaptor) with a hole in the center for the trans output shaft. That should stop most of the ATF from getting into the adaptor.
