I'm looking to buy that lower rear member brace.
ebay ad says for 1997 z
X308, I asked if it will fit my 2000 xjr. He didn't know.

Will it fit my car?

 

Explain to me what that brace does the A-frame is not doing already?
IMO useless for our type of suspension.

 

To eliminate a little body roll and stiffen chassis.
Will it fit my 2000 XJR?

 

No offence intended, as I like mod's ...
But in this case, realize that it is waste of money, as you will have to remove the original A frame, which does exactly the same already.
If you want ot stiff up matters, look at thicker sway bars.
And yes, I am 99% sure that a x300 and X308 have the same fitting points for the A frame.

 

The brace will have no effect on the body roll. The body roll is controlled, as Eric mentioned, by the sway bars and by the Adaptive Damping shock absorbers (if you have them).

I also agree that the brace will, most probably, not provide any additional stiffening of the chassis. I consider these braces just a fancy thing to have and only when they can be seen by the drivers behind you.