I'm not sure what axles you have.
I assume an HP Dana 30 in the front, but there were several options for the rear in those years.
As far as I know, the lowest gear sets available for the Dana 30 are 4.88's.
Also, and again my knowledge of the older XJ's is sketchy, but the axle shafts were quite week as well.
With only 115bhp, you definately need all the help you can get though.
If you end up doing this, please post your results.
My only advice would be to not have a heavy foot on the skinny pedal.
Mike