Well, I hated the original arm rests in my cherokee because they were broken. I went to the junkyard and noticed that all of the ones like mine that were there where busted. So I found the armrests that they had in the higher end models and swapped them in. They are basically bolt on except you have to cut a small hole in your door panel and add a retaining clip to the door itself.
So when you buy the new armrests you have to get the armrest those chrome plastic things (If they have them some didnt but I liked the way they looked) and the metal retaining clip behind the door panel (I am holding the clip in my hand in the picture). I would also highly recommend getting the screws for the handle as well because when I removed my old handles the screws were wedged inside them and I had a hard time getting them out.
Next remove the door panel. Its straight forward, just remove the old arm rests, window crank if you have it, and pop the plastic clips around the door (Just pull they will pop off). Once you have the panel loose slid the panel through the door handle/lock switch assembly and the panel should be off. Next install the retaining clip, I found a screw driver and needle nose pliers to be helpful.
Once the clip is in place make a small hole in your door panel where the clip is, I used a drill for this but it could easily be accomplished with a knife.
Reinstall the door panel using the steps from above in reverse order. Make sure to clip the top of the panel on before pushing the panel back on. Install the new armrest by holding the armrest vertically and pushing it through the hole into the retaining clip. Swivel the armrest to its horizontal position and make sure that it holds in place. Make sure that you put on the chrome panels if you choose to use them and install the two screws into the armrest. Do this for both sides and it should look like this.
I know its a simple thing to do but maybe you didnt even think about it and now you want to do it. I like these ones better as not only are they not broken but its a bit easier to close the door as well.