The differential is very sensitive to adjustment. 1 full turn is a lot, you may need only a 1/4 or less to feel a difference. If the diff is slipping despite it being at the correct adjustment the best thing to do is strip it right down, flipping the diff plates, clean and relube.
If that doesnt cure it then the only thing to do is a full rebuild, using the same outdrives if they are not too worn and renew the other components, period.
the diff spring, bolt, nut, thrust race and diff plates/balls are all important items on a rebuild, trying to save a few $ might result in the diff still slipping.... That said, make sure the diff bolt and nut are not stripped, it may be something as simple as that