U-joints on a non-CV driveshaft must be 'in phase' — both yokes in the same plane. Phasing errors cause vibration that no rebalance can fix.
On a conventional two-joint driveshaft (single-cardan at each end), the two yokes on the shaft itself must be in the same plane — 'in phase.' If one yoke is rotated 90 degrees relative to the other, every revolution of the driveshaft creates a velocity wave that vibrates harshly.
Factory shafts are built in phase. The problem arises after:
Visual check: lay the driveshaft on the floor. Both yokes should open in the same direction — like two parallel forks. If they're 90 degrees apart (one up, one to the side), the shaft is out of phase.
Fix: disassemble the shaft, rotate the slip yoke 90 degrees, reassemble in phase. Mark before disassembly. This is a 30-minute job with the shaft on a bench.
CV driveshafts (double-cardan at the t-case end) do not have this constraint — the CV joint cancels the velocity error inherently. This is one reason a CV driveshaft after SYE is more forgiving than a single-cardan replacement.
Vibration diagnosis order:
1. Phasing check (free, visual)
2. U-joint condition (any roughness in rotation)
3. Balance (last — driveshaft shops balance routinely)
4. Pinion angle (if SYE'd, see pinion angle entry)
Many XJ vibration complaints come from misphased shafts, not bad u-joints. Always check phasing first.
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.