The Facts Behind Differential Upgrades
Frame cracks around a Chevy's steering box. Closed-knuckle front-axle assemblies on early-'70s Fords. Model 20 axles on Jeeps.
Flaws. Every vehicle has at least one weakness. Others have quite a few. Some problems are easily fixed and don't cost an arm and a leg, but, unfortunately, most flaws aren't easy and can require a sizable investment to be remedied.
The Model 20 was used for 10 years under the back of all CJs and its weaknesses are infamous. Regular on-road-only use with stock-size tires does not usually aggravate the weaknesses, and the diff works just fine, but once you put bigger tires on a Jeep, rocky terrain underneath it, and some extra ponies under the hood, serious breakage is soon to follow.
A common fix and upgrade is to swap in another axlehousing-most often a Dana 44-and throw away the Model 20. But finding a Dana 44 in good shape is getting more and more difficult, and the cost of a completely rebuilt Dana can be enough to turn people away. As a result, some enthusiasts go with the enormous amount of aftermarket parts available for strengthening the Model 20.
In this article, we'll examine exactly what it takes to upgrade a typical Model 20 versus swapping in a Dana 44. We're keeping a budget in mind and will do a cost comparison to show you how the numbers add up.
To learn the pros and cons of both axles, we went to Tri-County Gear in Pomona, California. The company has been rebuilding axle assemblies for more than 10 years and can customize one to fit almost any application. The staff was able to give us some straight answers, and they had several axlehousings in various stages of assembly so we could compare internal parts.
The key to properly swapping in a Dana 44 is finding one that will work under a CJ. If you plan to hunt down a 44 on your own, be prepared for some headaches. Many trucks came equipped with the Dana 44, and there are millions of them out there, but there's a catch: There are countless versions. Some are narrow, some are wide, some have tapered (two-piece) axles, and most have different widths and spring-perch locations.
The ideal Dana 44 for this swap is one from a '72-'75 CJ because it's the correct width and has 30-spline, one-piece axles. The spring perches, however, will have to be relocated to fit under a '76-'86 CJ, a task that requires a skilled drivetrain shop like Tri-County. Also, some '76-'79 CJ-7s came with a Quadra-Trac T-case and offset centersections. These will accept a Dana 44 from a '70 1/2-'71 CJ-5 or CJ-6 with the Model 18 transfer case.
Other Danas can be used, but they usually have to be narrowed or otherwise altered, which adds considerable expense. Postal Jeeps came with Dana 44s and can be picked up cheap, but there are other difficulties with swapping one of these Danas under a CJ. The main problem is that the centersection is offset by a few inches. We've been told that a CJ at stock height will accommodate a Postal Dana, but once the Jeep is lifted, the driveline angle gets severe and the U-joints start failing. Postal Danas are also narrower, but this can be corrected with wheel backspacing. We'd stick with a Dana 44 out of a CJ.