Just oppinion, but a good approach may be adding a class three hitch front and rear and just leaving the stock bumpers.
My reasoning being, the stock bumpers, are by test (tested by my wife on numerious occasions) plenty strong enough.
The weakness is in the frame of the XJ. Not excactly weak, but built as a unit, adding a bumper and brakets, raröey takes into account the whole unit. The stock bumpers/brackets are designed to crush, before deforming the frame to a marked degree.
Adding 1/4 inch thick bumper brackets, to a frame thats in effect sheet steal, doesn´t make much sense.
Trailer hitches front and rear, add a frame insert (a nut strip) and spread the stress over a wider area of the frame.
If you decide to make your own bumpers a close look at trailer hitch mounting systems, will probably be a big help. Strong enough, without making it so rigid, a slow speed crash, crushes the frame.
The front and rear trailer hitches also make for a handy place to use a High Jack.
The rear trailer hitch, while costing a little clearance, protects the gas tank in a rear ender and helps protect the tank some, when draging the rear end in a ditch.