Jim, if you're replacing the diff pinion seal, be aware that the correct size is 1.625 (41.275mm)shaft, 2.75 (69.85mm) housing, and .5 (12.5mm) thickness. These are no longer available I think, so the next best standard size is 1.625shaft, 2.75housing and .375thickness. Shaft and housing are exact, the difference in thickness doesn't matter, and in fact gives you more choice of fitting position to avoid the wear groove.
I mention this because I have had the wrong sized ones sold as replacements from two different suppliers before, and in the end got one to size from a bearing supplier. I mentioned it to Martin and he was going to look into it.
Nut needs to be TIGHT, and I've Loctited mine for good measure!