1999 XJR.

When I shift into either drive or reverse, there is a slight clunking sound and the car seems to lurch either forward a bit when put into D, or backwards a bit when put into R. The sound is coming from the rear of the car. This is a new car to me and I plan on getting the transmission fluid changed as I don't know its history. Is this something that a simple trans fluid change could fix, or could it be indicative or something more serious?

 

Start with a fluid change and check for stored DTCs.

 

Could be a lot of things, collapsed transmission mount would be my main suspect.

If you’re close to chicago and need a dipstick for the mercedes transmission I can stop by and give you my spare.

 

Since the noise is coming from the rear of the car, I would check the whole driveline, especially the differential and drive shaft(s) u-joints.

 

I have question, you state it lurches, but then you state the sound is coming from the back? Are you saying you get a clunk noise along with a movement either forward or backward?
After the fluid change and if you still have the issue....a +1 to Marks suggestion. Here's a thread to review, but don't focus on the center bearing (though check it out). Look at the Jurid coupler in Post #3 and Post #13 regarding Universal Joints.

https://www.jaguarforums.com/forum/x...g-info-188400/

If you find you need the Jurid Couplings replaced, be sure to review this thread before attempting the installation....

https://www.jaguarforums.com/forum/x...pshaft-207717/

You can find these along with many other How To's in the ....wait for it ... How To section at the top of the forum. A number of those really need to be retitled, they are a bit long in the tooth and were the first Threads posted.