Earlier this year I successfully removed the failing 4.2L XK engine in our 1984 XJ6 Vanden Plas with a nicely running engine from my 1987 XJ6 parts car. While I had the engine out I cleaned up, painted, and otherwise restored the engine bay to look as new. That is when I removed and replaced the front subframe front and rear mounts on that car. I had done the front mounts before on three of my Jaguars and as expected it was a piece of cake to do. I had no problem removing and replacing the rear mounts with the engine out but I remember wondering “how in the world can you remove and replace these front subframe rear mounts with an engine still in there?”. There is one nut that you access from below that is easy, but there are two bolts that you access from inside the engine bay. I sat on the top of the front subframe to remove and replace those bolts and at that the were a bit tricky to get into place. I have no idea how I would have removed and replaced them with an engine still in there.
The rubber on both of these front subframe rear mounts had deteriorated badly and on one side the rubber had separated from the metal, so I was glad that I took the time to remove, inspect, and replace them on this 35 year old XJ6. The same rubber front subframe rear mounts were used on the pre-facelift XJ-S. I have no idea what the facelift cars used.
Paul