Most likely time for a new tranny, if I remember right these are sealed units so you can't service them normally.
Unfortunately, your Jag come from the days when Ford owned the brand. That being said, the trans might now be completely shot. It may be just one or two of the gears that are failing. DO NOT DRIVE IT. Have it towed to either a reputable mechanic or better yet, a Ford or Jag Dealer and have them pull the tranny and find out what the cost of the repair is. Its a very nice are although if you do some research on line, Jags, Range Rovers, and other British made cars have serious reliability issues especially as they age. Best of luck!
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