One thing that folks often overlook is flex in the unibody in the engine compartment area which stresses motor mounts. Most stock framed rigs have a crossmember under the motor, your XJ does not. On my last XJ, I fabbed a trackbar bracket brace/crossmember (RE now builds a bolt-on unit show below)
that tied the uni-frame rails together. I immediately noticed an improvement in steering response (less flex) and never had another motor mount issue failure in 80k miles, after replacing with original stock units.
On another note, my theory on the stock mounts is that they have a design flaw that leads to premature failure.The strap of metal that arches over the rubber mount does nothing to prevent stretching of the rubber to the point that the metal tube that the horizontal bolt passes through will tear free.
A simple solution is to buy some polyurethane roofing sealer from Home Depot
and before you install new motor mounts, pump the void between the rubber mount and the metal strap full of poly. Let them cure for a few days before you install. This will prevent the overstretching of the rubber without adding unneccessary vibrations that are inherent with full poly MORE-style mounts.
If you are going to run your rig in a JeepSpeed-like fashion, beefing up the mount plate that bolts to the engine block is a must too, in addition to the crossmember and beefed mounts.