Daihatsu Rocky drivetrain parts are plentiful and not too terribly expensive. The Rocky's rear axle has an offset differential section like the original axle in most older Jeeps. Its 3/4 floating design is stronger than the pencil thin full floating axle shaft found in Willy's MB/Ford GPW and the wear prone Woodruff key axle shaft & hub combo of the M38's semi-floating axle. There is a wide range of differential ratios as well as a limited slip diff available to choose from. The brakes are also better than stock Jeep brakes.
The front axle is chosen usually to match the rear, plus it comes with disc brakes, which are an added bonus. Axle width is also perfect for most narrow track older Jeeps, which can benefit from the slight increase in track width without looking overly goofy.
I have also noticed that there is a lot less driveline lash in Rocky axles as compared to Jeep axles, probably because they are usually in a lot better condition than the axles being replaced.
In addition, the Rocky's 5-speed gearbox is also a very popular replacement/upgrade for Jeep 3-speeds and 4-speeds. Some people also opt for the Rocky's transfer case because of its easier to operate and less noisy single stick design. However, any true Jeeper knows the Jeep's own transfer case is superior.