According to the internet
Jk wheels have 6.25 back space (depending on wheels, assuming 7.5 wide as most are)
Xj have 5.25 (depending on what stock wheels you have)
The jk wheels, with the spacer, will have a bs of 5.25
So they will be an outstanding 6mm or so further out
And the same inward.
Hopefully my maths is correct lol