I've been looking at a whole lot of how-to pinion bearing replacement stuff, and one symptom my diff does not have is any play in the prop shaft to suggest that the pinion bearings are shot.
That is, no side-to-side, no up-down or fore-aft play at the front diff end of the prop shaft at all. None. The only movement is on the shaft where the front section slides into the rear section of the prop shaft (is that called a slip joint?)
Just to confuse things a little more, I degreased the diff again and went for about 20km drive, speeds up to 80km/h. Got home and not even any weeping of oil on the diff.
I'll give it a bit longer, maybe it just takes a while to get leaking at the pinion seal...
If it's the pinion bearings, it looks like I can get a kit for about $100 and the inch-lb gauge for maybe $150-$200. Something like this?