Ugh! 
I had taken Superblue to my usual tire/battery, etc. national chain shop for her annual safety inspection and, this being a different shop location than my last visit, this time they actually attempted to check the lube level in the final drive. Unfortunately, the service manager discovered that his square head drive bit was too small to engage the check plug. Another tech had one, but it was way too big (why am I reminded of The Three Bears here 
?). Anyway, he opined that the lube level was indeed low, for the simple fact that the housing had clearly been leaking – he said old lube was all over the bottom of the housing and even showed me the tops of his forearms where some of it had rubbed off onto them while he was trying to get the check plug loose. 
I wasn’t aware that the final drive gasket has a history of leaking on our post-face-lifts, esp. @ only 85K miles (no telling how long it was going on before that). The service manager - after sitting down next to me - opined that I should look for a suspension specialty shop to take the cover off the box, r/r the gasket, put it back on and fill it up. He said it appears the only obstacle to that task is that there is some plate or frame piece that goes across, very close to the back side of the box near the bottom, which would prevent reaching the cover bolts . This would have to be removed first.
I told the tech I am worried now that the differential may freeze up at any time while going down the road, ruining things and maybe even causing me to lose control of my Jag., resulting in a crash. He told me that is indeed possible, but assured me I would hear a very disturbing sound quite some time beforehand from the back end whenever the lube level got to be THAT low. 
He did take Superblue for a short test drive (part of the safety inspection) and said he did not hear anything that sounded like warning noise from the box. 
So, I’m thinking of parking her and driving my XJ8 again until I can get the job done, as who knows - maybe some damage has already been done to the gears in the assembly by driving it so far with lube at a lower than normal level. 
If so, that would mean r/ring the rear axle to fix things (or have the differential gears rebuilt). 
On another note, the service manager also attempted to lube the 10 lubrication points on the rear suspension (4 of which are on the U-joints) shown in the Jag maintenance schedule literature. At first, he said he could not find any, but went back for a second look after I let him know that some of them are covered by rubber blanking plugs, which have to be removed to access the nipple fittings. He came back and said he had found one or two of them, and they were on the u-joints, but when he tried to Zerk one of them up with grease the grease just squirted out the sides of the fitting, meaning that it apparently was clogged up with road debris, etc… (I guess there’s nothing that can be injected to “clean” then out?). Anyway, he said he closely examined, felt and shook the u-joints to check them to make sure they were properly intact, which they were. He advised to not worry about them then until and unless they started making noise, and then r/r them.
I thanked him for his efforts … unlike the other tech at their other shop, he seemed knowledgeable, hard-working, attentive to detail, and, most importantly, IMHO, caring about the customer’s car, almost like it were his own. 
btw, what is “ie” short for?
) and in less than 30 days it blew out again in the same spot (the back end, in the area between the pair of half-moon seals.) 
The major chain auto parts store I bought it at, to their credit, said if I brought the remnants of it back to them and still had my receipt they would issue me a full refund, but unfortunately I had already tossed the receipt and they had no way to confirm that I bought it there or when I bought it. 
The second time around I went with Mahle for the gasket, and it is still rock solid today, several months after installation.