As a rule of thumb, if I'm towing a 4wd truck short distance, under say 50 miles, I just put the x-fer in neutral. Over that, at highway speed, disconnect the drive shafts from the difs. The reason being, most X-fer cases don't have a reverse oil pick up. Meaning most of the oil gets moved through the gears when power comes from the x-mission, but not the other way.