Rear Main Seal Leak
There's one answer for you. It seems that a lot of Jags may have this problem.
I, being who I am, would go get a Haynes manual for it, then jack the car up and take a peek for myself. If it is the real main seal, you might be able to limp it 100 miles (make sure you carry plenty of extra oil). Another idea is to get a UHaul tow dolly (if you or a friend has a truck to tow it with).
My Haynes manual only goes up to 85 on the XJS, so I don't know how much help it would be, but here's something I found interesting:
"An improved method of sealing the rear main bearing cap has been introduced. Instead of using 'hockey stick' type seals, silicone type instant gasket type sealant should be injected into the cap groove until it is ejected from B (Fig. 13.2)."
My scanner doesn't work, or else I'd scan the picture in for you. But it simply shows a caulk gun shooting silicon into some groove near the rear main seal.
Also, it says that it is possible to remove and replace the rear main bearings with the engine in the car.
"Renewal of the main bearing shells is possible without removing the engine, but it is unlikely to be required unless it is positively known that main bearing failure alone has occurred as the result of high mileage or a lubrication fault (oil leak, in other words). Normally, the crankshaft will also have worn, and will be renewed at the same time as the bearings during major overhaul."
Again, your best bet is to find a Haynes manual and look at the oil leak yourself. I hope any of this helps.