I now have my '89 XJS to a very good and meticulous (know Euro but first Jag) mechanic shop, recommended by 4 people. He showed me his All-Data master program with all updates, bringing up the XJS step-by step for all service and jobs. -I had given him a couple articles from this forum, including photos of the rear cage removed to show the inboard brakes.
I had bought Koni Shocks, new coils, rear rotors, pads and e-pads, diff axle seals, full tune-up parts, transmission mount bushings, and new coolant hoses. I know, "Wow". I had some money tucked aside, so it was the time. -
He did drop the complete cage, said it was only an hour, which really simplfied the brake work. My problem, as of him phoning tonight, one rear caliper is seized, and the diff axle seals as listed as correct oned from NAPA, are the wrong ones! The numbers on the original seals show no cross #, and the auto supply and bearing shops list them as Jag supply only.
I don't know if he can free up the original caliper or if I do need a new one yet, or where I can get a set of seals ASAP. I will probably try Paul's Jag as soon as he can be reached, unless a good member on here can source me easier (and closer) to Atlantic Canada.

 

Coventry West may be the fastest at identifying the parts for you and overnighting as they specialise in Jaguar rear ends.

The following link leads to a site where you might find the cross references:

DazeCars, Mustang IRS or Independent Rear Suspension page II

And this is just for drooling:

Independent Rear Suspension Forum / The Jaguar IRS picture thread

 

Coventry West, Terry's, Welsh, SNG Barrat can all supply what you need. I would buy a rebuilt caliper and be done with it. Even if you "free up" the caliper it will need to be rebuilt. By the time you buy the seals and pistons to rebuild it you have spent more money and certainly more time. If one caliper is frozen the other is probably not far behind. It most certainly is corroded internally. Brake fluid attracts water. You should flush the fluid every 2 years at minimum.

A quick search finds both NAPA and Rockauto.com having calipers for under $100. There is a pretty hefty core charge. You may want to source it locally so you aren't dealing with shipping the core back. Not sure if there is a NAPA near you or if Rock Auto ships to Canada but there is hope.