Regardless of whether your plans are to build a monster Jeep that can conquer any off-road terrain, or just to add larger wheels and tires for a more rugged look, your Jeep is only as strong as its weakest component. And stock driveline parts like transfer cases, axles, and drive shafts are often the first thing to suffer when suspension lifts, wheels and larger tires are added.
Your Jeep's driveline is pretty much your vehicle's backbone. It is componetry from the engine on back that helps send power to the wheels to keep the vehicle moving. We're talking parts like the transmission, transfer case, driveshaft, axle shafts and housings. Left alone as stock they do a more than adequate job, but when stressed following vehicle modifications such as suspension lift kits or larger tires, they easily become under powered or worse - broken.
How you plan to drive, once you've made these modifications in lift or tire size, will ideally determine if you just need to just beef up existing driveline components or look at heavy-duty replacements, such as replacing your driveshaft. For many, adding a lift and larger tires is simply a way to accomplish that rugged look with the occasional off-road experience. Beefing up parts like the transfer case with a Slip Yoke Eliminator kit as well as swapping to 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 can help support the stock axle shaft when larger tires are added. Another thing to look at is the axle itself and changing out the seals and bearing, or even strengthening the ring and pinion with a heavy duty performance set.
However, you can only beef up driveline components so much before you max out their capability and need to start looking at replacements; especially if your plans are for moderate to severe off-roading. One of the first changes people make is to the vehicle's differential by either adding lockers to gain better traction off-road, or by changing out the ring and pinion gears to help restore the power lost following a tire upgrade. Depending on the amount of off-roading, you may even want to upgrade the 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 those larger tires in all off-road conditions.
Of course, there are also plenty of replacement accessories available as well such as shifter boots, seals, fluids, gaskets, and hardware which will cover pretty much all types of driveline repair projects.