I've haven't heard of any issues installing lower gears in a rear axle with 2.35's. However, you'll need to change the rear carrier for gears over 3.54. If you have a non disconnect front axle, it's my understanding that 3.54's will work. But, AMC did some funky things to those 2.35 front axles so I'm not sure. For the front you would want a ring and pinion set for a CJ7 Dana 30. 3.54's not 3.55's. Rock Auto has 3.55's for the rear for 102.79.
A jeep XJ rear axle can be bought for about the same as the gears alone. If you can install the gears yourself and can also do an axle swap that requires a little modification of the shock mounts, then it's a wash. I don't have the expertise or tools to install a ring and pinion set, but an axle swap was within my capabilities, so that's the route I went. A rear axle cost me $100 and my labor was free. Also, I put in a Chrysler 8.25 which is much stronger than the AMC model 15.