If you are set on keeping the XJ I would just get a dog barrier for the back. One that extends up from the top of the rear seat to the roof. This will create a "crated" area for the dog if you are in a spot where you want/need to restrict their movement. (like keeping them out of the beef jerky you left in the front seat.

) Just like the crate, the small space is stress relieving for the dogs. My two (Australian Shepard and a Wolf Hybrid) just chill out and sleep anytime they are in the "crated area" of my SUV.
These take up much less space than having an entire crate. Usually easy to take down, or just fold the seat down if you want to give the dog full access.
Be mindful of how and where you park, make sure the back is in the shade, open the windows, etc. I usually toss up my sunshades even in the dead of winter just to limit who can see the dogs and what the dogs can see. Alos makes it a bit darker and more relaxing.
FWIW, my two dogs go with me EVERYWHERE!!
If you are up for changing out of the XJ I would suggest an extended cab pickup with a topper. Easy to have the dogs in the cab or the bed/topper area whichever you prefer. I just picked up a truck in this format specifically with the dogs in mind. Mine are very well trained and are fine left in the cab area. Usually just sleep behind the seats. But this gives me a a couple of options. WET dogs can go in the bed area!! Or, I can put any food items in whichever area the dogs are not going to be in at the moment.
If you have not I would suggest a high quality window tint. Does not have to be dark as long as it reduces lots of heat. Dark is kind of nice because the dogs pay a little less attention to the outside world and the outside world pays a little less attention to the dogs.