The AMC model 20 rearend has been getting a bad rap for years. The two-piece axle design that AMC used in most Model 20's is about as sturdy as using dental floss for a tow rope. For reasons of economy or otherwise, AMC stuck with its two-piece design axle right up until the very end. There are some applications that come with the one-piece axles stock, but they are few and far between. Given these drawbacks, you are probably thinking it is time to retire that Model 20 rearend. Not so fast. The two-piece axles are a weak link that can easily be overcome. Also, the ring & pinion in a Model 20 is actually larger than a Dana 44, so this larger gear can take more abuse and holds up well with tall tires.
There are many sources for high quality one-piece replacement axles that are superior to the original design. I prefer designs that use Timken SET9 axles bearings. These high quality bearings do not need to be adjusted for proper end play. They hold up very well with oversized tires, and replacements are readily available if they become damaged by rust or wear. For those of you to whom cost is no barrier, Warn Industries now makes a full float axle kit. These high quality kits use forged 29 spline axles and include all of the hardware to make you M20 a full float rearend.
As far as axle strength goes, the M20 axles are 29 spline and are virtually as strong as the 30 spline D44 axles. I have seen axles break in both rearends, so neither is completely indestructible. However, both are relatively strong and hold up well to all but the severest conditions.
Upgrading to stronger axles is only the beginning of a strong rearend. I also recommend a good positraction or locker and the right gear ratio. You may say, "I told you this diff is a loser," but any other design will also need a good traction differential and the right ratio. For occasional off-road use, the stock Trac-Lok limited slip may work, but I seldom recommend it due to its wimpy clutch design. Auburn Gear now makes a strong, aggressive limited slip that works well off-road and drives fairly smoothly on pavement. At one time, AMC used Powr-Loks in some passenger car applications. If they can still be found in wrecking yards, these work great for off road. There are also several locker available: Lock-Rights, EZ Lockers, ARB Lockers, and of course, the Detroit Locker. I personally like the strength and ease of use of the Detroit Locker. However, the ARB Airlocker is smoother for street driving, and the Lock-Right and EZ Lockers are easier to install.
When it comes to gears, the Model 20 ring & pinion gears are larger that those of the Dana 44. The D44 is ring gear 8.5" in diameter and the M20 is 8.875" in diameter. The pinion shaft on the M20 is also much larger and supports the pinion teeth very well under heavy loads. The only drawback of the M20 ring & pinion is the lack of numerically high ratios available. To my knowledge, 4.56 is the lowest (highest numerically) ratio available. A complete list of ratios that are available includes: 2.73, 3.07, 3.31, 3.54, 3.73, 4.10, and 4.56.
So, yes, there are some shortcomings when using the Model 20 rearend. Nevertheless, when all is considered, I still believe that the Model 20 is a good choice in most situations, and the bad reputation earned by its weak axles is not reason enough to pitch the entire rearend.