The rear driveshaft carries power from the transfer case slip yoke to the axle pinion. Worn u-joints and a sloppy slip yoke cause vibration, clunking, and a leaking transfer case output seal — service the joints or step up to an SYE and CV shaft on a lifted rig.
The stock XJ rear shaft is a conventional 1310-series single-cardan shaft: a u-joint at each end and a splined slip yoke that slides in and out of the NP231 or NP242 rear output. Trouble shows up as a vibration that builds with speed, a clunk when you shift into gear or get on and off the throttle, or fresh gear oil weeping from the back of the transfer case. Worn rear u-joints are the usual cause. On lifted trucks the slip yoke rides deeper on its splines, drags a corroded section of shaft across the output seal, and starts the case leaking — the reason a slip yoke eliminator (SYE) with a double-cardan CV shaft becomes the real fix past about 3 inches of lift.
Steps
Chock the front wheels, set the parking brake, and get the rear axle safely on stands with the driveshaft accessible. Slide a drip pan under the transfer case tailhousing — pulling the slip yoke lets gear oil out.
Mark the shaft-to-pinion relationship with a paint marker. The shaft is balanced and goes back in the same clock position, or the vibration returns.
At the pinion, remove the u-joint U-bolt nuts (or strap bolts on later trucks). Tape the bearing caps to the u-joint so they don't fall off and spill their needle bearings.
Slide the shaft rearward off the pinion, then pull the slip yoke straight out of the transfer case. Expect gear oil — have the pan ready and plug the output or lift the tailhousing end high to slow the drip.
Inspect: grab each yoke and check for radial play in the u-joints, then feel the slip yoke splines for wear and the sealing surface for rust and pitting. A grooved or rusty slip yoke surface is what chews up the output seal.
To service the joints: remove the snap rings, press out the old u-joints with a press or a vise and sockets — never hammer the caps, it wrecks the needle bearings — and press in fresh greasable Spicer 5-153X joints. Seat both snap rings fully.
If the transfer case was leaking, replace the rear output seal now while the yoke is out. A pitted slip yoke will destroy a new seal, so replace the yoke too if the sealing surface is rough.
On a lifted rig with chronic vibration or a repeat leak, this is the point to install an SYE and a built-to-length double-cardan CV rear shaft instead of reusing the slip-yoke shaft.
Reinstall the slip yoke into the transfer case, seat the shaft on the pinion in the marked clock position, and start all fasteners by hand. Torque the pinion U-bolt nuts to 14 ft-lb (strap bolts to 16 ft-lb), working in a cross pattern.
Top off the transfer case fluid to the fill plug, grease the new u-joints through the zerks, and road test. A residual vibration usually means the shaft went back off its mark or pinion angle needs correcting.
Why it works
Fresh greasable Spicer joints run under $40 in parts and kill the vibration and clunk
Replacing the output seal and slip yoke while the shaft is out stops the transfer case leak at its source
An SYE and CV shaft cures driveline vibration on a lifted XJ and ends slip-yoke seal leaks for good
Trade-offs
Pressing u-joints needs a press or a vise and patience; done wrong it ruins the new joint
Pulling the slip yoke drains transfer case fluid, so plan on a refill and have the right gear oil on hand
An SYE is a bigger job and cost — it means dropping the transfer case tailhousing or swapping the output shaft, not a quick fix
Tools required
socket set
torque wrench
u-joint press or vise + sockets
snap ring pliers
paint marker
drip pan
soft mallet
Parts
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Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.