I think that before I embarked upon this, I'd want to maybe eliminate a couple of bits. I'd certainly drop the rear prop, lock the centre diff and go for a spin. Having had various diff whines in the past, several of which Jon has cured, I'd want to know that absolutely the cause. I had exactly what you are describing - the on and off the power noise. Rear diffs rarely give rise to problems on these mighty monsters. It might be worth asking Karl just how much a diff might be. I know I'd rather swap a diff then swap that bearing. Either way, it's very unlikely to let go or anything hideous. You may not have the dashboard centre diff lock option of course at the moment so doing a road test as I have described might be tricky. I would factor in fitting the switch in the dash as a priority. For any 80 owner really.
Chris