the advantages are mostly in climbing ability, but the XJ at 101 is already at a pretty good wheelbase for 37s down.
if you move the rear back don't do it by redrilling the leaf, do it by redrilling the spring perch if you're only looking for an inch or two, or flipping the leafs if you're looking for several inches. flipping the springs will likely necessitate a traction bar. the main concerns with moving the rear axle are gas tank clearance and wheel-well clearance.
for moving the front you will need longer control arms, but you will also need to move the trackbar, steering box and coil mounts if you move it much. fender clearance would be easier to deal with on the front than the rear.
