The gearbox output shaft splines as well as the rear drive splines should be greased with a specific spline grease which is extra clingy and wont get spun off like what you found inside your shaft tube. Many here like the Wirth 3000 spline grease . I have been using BMW's brand from a local shop. Also, a marine outdrive spline lube is excellent for this application. Don't forget to grease both inner and outer sides of the hollow spline adapter on the rear drive end.