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Some basic axle information:
Stock Axles:
Dana 30 Dana 30s are available in both high pinion and low pinion versions. The Dana 30 is actually surprising stout for a small stock axle. Dana 30s come factory with 27 spline shafts, and can handle a 33" tire. Usually a 35" tire is pretty safe as well if you're using aftermarket axle shafts.
Dana 35 The Dana 35 is a weak axle, I would consider a 33" tire the upper limit of this axle. Its not if this axle will break, its when. These axles can sort of handle a 35" tire if you upgrade the shafts. You should generally do your best to avoid putting any money into this axle. My example images: Broken 35 / Broken 35 Shaft
Dana 44 Dana 44s are available in both high pinion and low pinion versions The Dana 44 axles can be found in the rear of some TJs and in the front of the Rubicons. The Rubicon versions do have slightly weaker housings though. Most 44s were 30 spline axle shafts. You can safely run a 37" tire on a 44.
AMC 20 AMC 20s can be found underneath early CJs. They have completely round covers. Although they have a ring and pinion nearly the size of a 44, they still aren't very desirable. The weak point on this axle is the two piece axle shafts, upgrading to a one piece axle shaft will greatly improve the strength. Many have good luck wheeling this axle with one piece shafts and a truss. Still keep in mind it still a small axle and won't hold up if you're crazy on the skinny pedal.

Axles Commonly Upgraded To:
Dana 44 A good upgrade for many Jeeps particularly as a front axle. Again stick to a 36"-37" tire.
Dana 60 Dana 60s can be found with 16,23,30,32, and 35 spline axle shafts. Many are full floaters which is a strong point. These are larger axles, and hang down a little low, but who cares when you can safely run a 38" tire.
Ford 8.8 The Ford 8.8 can be found under nearly every car Ford makes. Many try to get them out of the late 90s Explorers as they had disc brakes, 31 spline shafts, and the larger 7/8" cross pin. The 8.8 out of an Explorer will also be almost stock width for a Jeep axle, and has the same bolt pattern for the wheels. 37" should be considered a maximum safe limit.
Ford 9" The Ford 9" is also a fairly common axle, and is slightly stronger than the 8.8. The 9" does not use c-clips like the 8.8. They came factory with either 28 or 31 spline shafts. There are lots of tricks to making these stronger, so if going this route, search around. I'd also give this axle a 37" tire limit on a stock setup.
GM 14 Bolt 14-bolts can make 60s look small, they are massive. Available in 30 and 33 spline axle shafts. This is a good strong axle for big motors and big tires. 14-bolts hold up to a 44" tire pretty well.

Those are the more popular axles, so I'll leave out things like Rockwells and Unimogs.
Related Topics: | Full Floating vs. Semi Floating Axles - | What's a high pinion axle? | Axle Swaps: |
Related Writeups: | Axle Identification |

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