Your Jeep Wrangler JK's driveline is pretty much considered the vehicle's backbone, and is made up of componetry from the engine on back that helps send power to the wheels to keep the vehicle moving. We mean parts like the transmission, transfer case, driveshaft, axle shafts and housings. Unmodified, they do an adequate job. But following installation of things like suspension lifts or larger tires to the Jeep, that driveline can easily become underpowered or worse - broken.
Regardless of whether your modification plans are to build a monster Wrangler JK to conquer off-road terrain, or just to add larger wheels and tires for a rugged look, your Jeep is only as strong as its weakest component. And how you plan to drive, once you've made these modifications, will determine if you just need to just beef up existing driveline components, or look at heavier-duty replacements.
Many JK owners utilize their vehicle as a daily driver, but still want a rugged look so they choose to add larger tires and a lift kit. So beefing up parts like the transfer case with a Slip Yoke Eliminator kit as well as swapping in a stronger drive shaft can help reduce driveline vibration and improve driveline angle which makes for a smoother ride. Other parts like replacement wheel hubs and bearings help add support to the stock axle shaft following installation of larger tires. One other modification can be the axle itself and changing out seals and bearing, or even strengthening the ring and pinion with a heavy duty performance set.
However, you can only shore up driveline components so much before you max out their capability. If your plans are for moderate to severe off-roading, you should look to replace your JK driveline parts with heavy duty alternatives. One of the first changes JK Wrangler owners make is to the vehicle's differential by either adding lockers to gain better off-road traction, or through upgrading ring and pinion gears to help restore power lost following a tire upgrade. Depending on your off-roading severity, you may even want to upgrade axle assemblies to a stronger Dana 44 or extra-heavy duty Dana 60 as these do a great job holding up to the abuse of larger tires in all off-road conditions.
There are also plenty of replacement driveline accessories available as well such as U-Joints, seals, bearings, fluids, gaskets, and hardware which will cover pretty much all types of driveline repair projects.