I agree the manual is vague and I understand the reason for the question now that I think about it.
You could try the shift High to Low or Low to High while stationary and use neutral in the transmission and if it shifts fine then you have a good place to start.
I would think that you would want a low amount of "energy" transferring between the engine/transmission and the transfer case to have a smooth shift. If the transmission is in drive, the output shaft will spin if the resistance is eliminated from it by engaging neutral in the transfer case. The standard transmission deuces recommend depressing the clutch and then shifting the transfer case. Leaving the transmission in gear prevents it from freeewheeling during the shift and possibly grinding the gears of the transfer case.
My suggestion would be to not shift into low range unless you are going slow (Obvious I know), and also I would put the transmission in neutral and make the shift smoothly but fairly quickly into the lower, or higher range. Then re-engage the transmission.
You will find out quickly what the setup likes and does not like, and then you can share your knowledge with others with the same question.
RL