Hmmm. I have never seen Dana axles with the shims outside of the bearings. I agree, that would make it difficult to get in without a spreader.
You can do this simply if you want. But you need a friend, and if you find any other problems, you will want to stop and get some experienced help.
If everything has been working well in your jeep so far, I would keep it simple. I would just remove the cover off the diff. Then I would check a few things. Put the transfer case in neutral, or remove the driveshaft.
1) Grab the big ring gear and see if it is in there securely. It should not move left to right.
2) Grab the pinion yoke at the front of the axle. It should spin, but not wobble.
3) With your friend holding the pinion yoke, you can check the free play between the ring gear and pinion gear. you move the ring gear back and forth, and you should only be able to feel a little bit of movement between the gear teeth. (Actually like .006") not much, but you can feel it.
4) If all that seems OK, and you don't see anything cracked or broken, then you have just completed a quick simple inspection. Now you can work on replacing that seal.
Replacing the seal.....
1) Remove the axle shafts and the differential the way I mentioned earlier with a friends help.
2) Look to be sure there are no shims on the outside of the differential bearings. If there are none, then you will not need a spreader later.
3) Do a quick visual check of your bearings. If everything looks smooth and snug, then you do not need to replace them.
4) Remove your faulty seal. Knock it out with a broomstick handle that you cut the broom off of. Or something similar.
5) Install your new seal. You can do it the way others have mentioned, or if you are carefull, VERY careful, you can find a big socket or something that fits just right. Something that you can find that you can tap with a hammer that is round and will rest on the metal part of the seal, so that when you tap it with a hammer, it will go in straight. The long broomstick may help you drive it in straight from the other side. You can also talk to someone at a parts counter, and they will show you some cheap tools to install seals with.
6) Reinstall your differential. If there were no shims outside of the bearings, do it the way I mentioned before, or...... if there were shims outside, get somebody to help you locally, because you are either going to have to get a spreader or relocate the shims.
Bottom line.... If everything else is in good shape, you will not have any problems. But if you are uncomfortable, or can't really visualize what you are doing, then get some help. If you stumble across parts that you think need replaced, you may want help.
Defination of "help"......
I would find someone or a shop that works on off-road vehicles, Jeeps , rock crawlers etc. They work on lots of Dana Axles, and would enjoy your jeep.