i have been battling a cracked driver side subframe for two years.
i purchased the JCR winch bumper with tie ins, once isntalled it stiffened my front end considerably.
i added a M.O.R.E. steering box brace, drilled a larger hole in the passenger side tie in and bolted both brackets together. night and day difference
then i installed an RE trackbar/bracket combo, then added the brace a week later, another great improvement over the bumper+box brace.
my front end is nice and steady now, i do not know how this will affect your xj farther down the body