The glove box should "deploy" slowly when the chrome button is pressed. It shouldn't drop suddenl'y. No idea how that is accomlpished on the XJ, but it sounds broken in your case.

 

I had exactly the same problem! Yes, there is a "damper". It's a plastic tube with a spring and piston in it, and it's mounted sideways up and behind the glove box. There's a piston inside the tube to which a string is tied, and the other end of the string is attached to the right side of the glove box. You can see it if you remove the cover that sits below the glove box. The actual damper unit is not that expensive ($20-$30), but it looks like a real pain to get at if you need to remove it. In my case, the damper end cover on the right side (the side that takes the spring pressure) had come out. I was able to press the spring and end cap back into the tube, and it's holding for now. So, have a look at yours and see if your string is still there and/or the end cap has come off.
Good Luck!
Greg
'04 XJ8