In my experience, diff pinions only leak when pinion bearing preload has failed and the pinion starts to move around in the seal.
Pinion seals are so well protected from debris and see no real heat, they should seal forever.
Certainly on every diff I have pulled apart, the leaking pinion seal has been accompanied by no pinion bearing preload or damaged bearings. My XJS was no different, needing a new front bearing race and new crush sleeve.
This would probably be an explanation for Doug's findings above.
I personally don't agree with the factory manual (and other vehicle manuals also reccommend to do it) suggesting to retourque to same position. That is pointless on any differential that has been in service, as wear has most definitely occurred and bearing wear is like a bell curve, lots initially as things take a set, long trouble free mid service life, then accelerated wear as things get loose.
As the only way to correctly set pinion preload is by measuring rotating torque.....well, the diff should come out to do it properly.

 

So, your 20 year old Jag has one side of the rear diff slightly out of spec on end play, and after 30.000 miles the tires are wearing on the inside?
Rotate the tires more often, and keep an eye on the rear diff and suspension.
I do not see any reason to spend 3 grand.

 

How often would you suggest that I rotate them? Right now I rotate the tires every other oil change (so 7000 miles, and that's on 70,000 mile high-end tires) and only this last time have the rear ones started showing this wear and it's only about half-way until time to rotate them again (so the wear has only shown up in about the last 3500 miles).

I agree and have no intention of spending 3k, Right now I am thinking the best plan is to buy one of the $300 rebuild kits and find a shop that can install it properly at a reasonable price even if I have to drive to where they are located and spend the night in a motel. Right now looking for suggestions on such a shop from the other members who may have experience with a shop that does good rebuild kit installs.

 

I rotate my tires about every 5000 miles. I agree, wear that has shown up in the last 3500 miles is a concern but I also see you said it is barely noticeable. Was nothing else found in trying to align the rear? Is this car your daily driver? I am thinking that rotating every 3-5000, and living with the idea of some tire wear is acceptable for a weekend car, if a daily driver maybe not.