Measure from t-case yoke center to pinion yoke center at ride height. Account for plunge (1-1.5" minimum) for suspension travel. Tom Wood's has a worksheet.
After SYE or lift, the rear driveshaft length almost certainly changes. Measure correctly the first time or your custom-built shaft will bind or pull out under articulation.
Measurement procedure (rear shaft):
Truck at ride height on level ground, fluids full, weight as it normally drives.
Place a tape measure from the center of the t-case rear output flange (after SYE this is the new yoke face) to the center of the pinion yoke face on the rear axle.
Note the measurement to 1/8" accuracy.
Subtract the typical yoke length convention used by your shop. Tom Wood's measures center-to-center and accounts for joint depth internally; specify their procedure if ordering from them.
For articulation (lift, droop, flex):
Stuff one tire fully and droop the other; measure again. The difference tells you required plunge.
Most XJs need 1.5-2" plunge for daily-driver lift; 3"+ for hardcore wheelers with long-arm setups.
Common errors:
Measuring on a lift with axle hanging: this measures droop position, not ride height
Measuring before SYE install: t-case output position changes ~2" between slip-yoke and SYE
Forgetting to subtract for joint length: get a worksheet from your driveshaft vendor
Front driveshaft: same principle but measure from t-case front output to front pinion yoke. CV joint at the t-case end is the OEM configuration; replace in-kind unless going to a custom front shaft for SAS.
Tom Wood's, Adams, and local driveshaft shops will build to your number. Confirm twice — a shaft built 1" too short is scrap.
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.