I was federally certified as a child safety seat installer, so I can provide some answers for you.
The safest way is to install it properly.
IIRC, the Wrangler's rear seatbelts use a lightweight locking latch. This may be sufficient without having to also install an external locking latch, though you won't know for certain until you test fit the seat. The general procedure is as follows:
1) place the seat in either of the rear positions.
2) route the belt through the seat and buckle it.
3) with your knee (and body weight) pushing the safety seat into your Jeep seat, pull the belt
snug.
4) with the seat empty, give it a few
vigorous shoves side-to-side (across the rear bench) as well as front-to-back at the bottom of the safety seat (toward and away from the Jeep's front seat). This is the moment of truth because your shoves are simulating crash forces, so don't wuss out--
shove it. You should see less than 1" of movement in either direction. If there is more than 1" of movement either way, the seat belt is not holding it snugly enough--return to step #3.
4a) some safety seats will hinder pulling the belt snug because the seat belt buckle (the female end) is too close to where the belt routes through the seat. If this is the case, you may rotate the buckle 360 degrees. This will twist the belt and cause it to shorten a bit. If the belt is really long, you can even give it a second full twist (for 720 degrees total) to shorten it further. Then return to step #3.
5) Once the seat is snugly attached to the car, you should be good to go. However, here is your final step:
Every day before you first put your child in the seat, give it another shove test to verify the seat is still snug. If you get more than 1" of movement in either direction, re-tighten the belt before putting your child in the seat.
A lightweight locking latch should maintain tension more-or-less indefinitely. However, if your seat constantly comes loose and requires re-tightening, then you ought to install the external locking latch. Federal law requires every car seat to include one, and it also requires the seat to have a provision to hold this latch so it is always available when needed. If you do need the external latch, be sure to install it correctly--it goes
right next to the seat belt's latch.
Other questions? Post 'em up.