I would not use the tiedown hooks. They probably aren't strong enough. If you need to tether the seat to the floor, you should probably drill through the cargo area floor (making doubly sure, of course, not to hit the gas tank, etc.) and install a proper anchor.
While some child seats work without a tether, I'd be hesitant to use one on the back seat of an XJ because I don't know if I'd trust the seatback to stay latched. That might be all right, especially if your XJ has shoulder belts and the seat utilizes them, but you should probably check with the seat manufacturer/instructions on that. You might also be able to get away with using the shoulder belt mounting point for the tether, since the point of the tether is simply to keep the seat from flipping forward, and it needn't be super snug in a straight line. Again, I'd expect the seat instructions to help with that question. If your XJ doesn't have shoulder belts in back, the mounting holes are still there under the trim, but the reinforcements are not, so you'd have to do some reinforcing. Still, if the angle is good enough, that might well make for a better anchor point than the floor, less likely to be disturbed by cargo. I don't think you could run a tether to the anchor points from the center seat position, though.
It's been many years since I've needed to do this (my "littlest" one just turned 21!), but as I recall, the anchors for these are not as hefty as regular seatbelt anchors, but they still need to hold up to a few G's. This is one area where a good old fashioned sedan has the edge. My 72 Mercedes still carries its history: 3 tether anchors in the package shelf!