I ask because I know the front had to be changed when AMC went from the V-6 to the inline 6. The radiator support is definitely different for the first years. I suspect the fenders are different, and if the fenders are different then the fender liners may well be too.
What year XJ are you working on?
I may be able to pull a fender liner from my local JY if you are okay with paying whatever they charge me, plus shipping.