Load-Dependent Driveline Vibration After a U-Joint Replacement

What is eliminated: anything that is purely a function of road speed and still rotates in neutral - tires, wheels, wheel bearings, brake rotors, and a plain shaft imbalance. All of those keep spinning at 90 km/h in neutral, so if they were the source the vibration would persist. Road-force balancing already removed tires as a candidate.

What is left - load-dependent faults. In neutral, torque through the driveline drops to near zero even though the shaft keeps turning. So the fault only appears when torque is applied:

• U-joint binding or incorrect phasing - a joint that is stiff, dry, or installed with the yokes out of phase will bind under torque and go quiet when unloaded.
• Driveline working angles - a shim removed from the centre bearing, a sagging or worn centre support, or a rear leaf/block change alters the angles. Angle-induced vibration is very sensitive to torque because the pinion climbs under load.
• Pinion bearing preload or depth from the seal replacement - too tight or too loose a pinion will vibrate and whine under load.
• Engine/transmission mount loading over - already partly excluded, but re-check under torque (power-brake test).

The rear U-joint and pinion seal work 2,000 km ago is the strongest lead, because the symptom dates from it.

Phasing. On a shaft using single Cardan joints, the yokes at each end of a given section must be in the same plane. If the shop pressed the joint in, or split the shaft, and reassembled it 180° out or one spline off, you get a second-order (twice-per-revolution) vibration that appears under load. Check the alignment marks, and physically sight along the shaft comparing yoke orientation. If the shaft has a slip yoke, confirm it is splined correctly and not one tooth off.

Runout. Support the shaft on V-blocks or check on the vehicle with a dial indicator at the centre of each tube section, not at the weld. Typical limit is around 0.010 in (0.25 mm) total indicated runout. Also check for missing balance weights, dents, and a worn or collapsed centre support bearing and its rubber insulator.

Working angles. With an inclinometer or digital angle finder on a level floor:

• Measure the transmission output, the front and rear shaft sections, and the pinion flange.
• Each joint's working angle should be small - roughly 0.5° to 3°.
• The front and rear working angles of a section should cancel within about 0.5° of each other.
• Correct with centre-bearing shims or, if the pinion angle changed, by correcting the suspension or using tapered shims.

Do the cheap checks first: phasing and centre bearing. They take minutes and explain a vibration that started right after a U-joint job.

Separate the shaft from the axle with a simple removal test.

1. Remove the rear driveshaft (mark its orientation first), shift the transfer case to a position where the truck can be safely pushed/rolled or run on a lift, and check the pinion flange: rotate it by hand and feel for roughness, notchiness or end play; measure rotational torque (pinion preload) against spec.
2. With the shaft on the bench, feel each joint through its full range - a binding or stiff joint with no grease passages is a failure even with no visible play. Confirm the joint is the correct series and that the snap rings are fully seated.
3. Compare the vibration before and after the removal test: if the vibration disappears with the shaft out (and the axle is spun separately), it is the shaft.

Pinion-specific signs: noise on coast/decel as well as acceleration, a leaking or weeping new seal (overfilled or a damaged seal from installation), and evidence that the pinion nut was reused or over-torqued, collapsing a crush sleeve and changing bearing preload. That is a differential repair, not a driveline repair, and the customer should be told the seal job is the likely origin - with documentation, since the other shop's work is the proximate cause.