Jaguar dealers have a very sophisticated vibration analyzer. If I were you, I'd bite the bullet (and this comes from an owner who has experienced nothing but ill-service from 5 different Jaguar dealers since buying my XJR new, and who has not darkened the doorway of a Jaguar dealership for 6 years), and have them diagnose it.....
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1999 XJR

 

I would first start with making sure my tyres are balanced. I have had the same problem once and balancing them did the trick. I doubt it's anything big like joints though on my previous car (2001 Jag XJ8 Super V8) I did have to change a bearing at 60K
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Ford Expedition history

 

Tim, based on your description of the problem, I will lean toward the drive line. But like Markus recommended, bite the bullet and have it checked out. You can spend a bunch of money changing things out to find a problem like that.

 

i wud check the u joints.. and rear wheel bearings.. and check the lower fulcrum bearings.. the casing likes to explode if never serviced.. depending on mileage

 

engine mounts leak when they go.. and xjr drive shafts are usually good