This is a sight familiar to Jeep owners. The stock slip yoke and short driveshaft are well-known limitations of the Jeep drivetrain. This particular transfer case has over 130,000 miles on it and is starting to show its age.
The NP231 transfer case is a pretty standard piece of off-road hardware. They came factory-installed on all kinds of small trucks, SUVs, and probably most famously, the Jeep Wrangler. Although these basic transfer cases are perfectly adequate for stock vehicles, not many off-road vehicles are left in factory form very long. Big tires and lift kits stress the internal components of these transfer cases far beyond what the factory ever intended. These additional loads, combined with the effects of age and high mileage, can lead to some pretty nasty failures.
Rockland Standard Gear (RSG) remanufactures transfer cases and manual transmissions, specializing in permanent fixes for flawed factory designs. RSG even offers custom all-wheel-drive-to-four-wheel-drive conversions. Applying this experience to the NP231, RSG remanufactures a standard NP231 but replaces the Morse chain, gears, and shafts with heavy-duty pieces. The company swaps the stock three-pinion planetary for an extremely beefy six-pinion unit and installs a forged Low-range selector fork.
To help cure extreme driveshaft angles that result from suspension lifts, the HD231 transfer case comes with a slip-yoke eliminator installed. Since this will require a new CV driveshaft, we ordered a complete driveshaft from 4 Wheel Parts. One of the benefits of eliminating the slip-yoke is that you can simply remove the rear driveshaft without losing the transfer-case oil and drive in front-wheel drive if you have a rear-axle failure.
With the swap done and the Jeep back on the road, we noticed a few things. The first major improvement was the lack of slop in the drivetrain, mostly due to replacing 130,000-mile-old parts. The bolt-on-style yoke and CV driveshaft allowed us to remove the skidplate drop pucks that were installed with this Jeep's lift kit, giving us another inch of center clearance, an improvement we noticed instantly on the trail. The first time we shifted it into Low range, we were shocked how much easier it was to get the gears to engage. We never realized how much flex there was in the stock shift-fork design and how severely it impacted range selection performance.
We completed our swap at Off Road Werkz in Ventura, California, but with a little patience, a couple of floor jacks and some jackstands, you could easily do this in your driveway. With the lift and air tools, it took us about an hour, so figure a little more time if you've never done a transfer-case swap before. There are a couple of steps that are immeasurably easier with an impact wrench, so you should probably rent or borrow one if you don't have one already. Be sure to check out the comparison photos of the internal components - the difference is staggering.
We supported the transfer case from the rear and removed the transmission mount and isolator bolts. We also supported the skidplate, removed the six bolts holding it to the frame, and lowered it away from the Jeep. | 
We then drained the transfer-case oil and removed the speedometer drive on the rear output housing, the vacuum switch on the top driver side, and the vent hose on the top center of the case. | 
Next, we unbolted and removed the rear driveshaft entirely (the front driveshaft only needs to be unbolted at the transfer-case yoke). The shift linkage pops out with a little help from a prybar. |