Ford 2003 Mustang Workshop Manual page 268

WARNING: Do not balance the wheels and tires while they are mounted on the vehicle.
Possible tire disintegration/differential failure could result, causing personal injury/extensive
component damage. Use an off-vehicle wheel and tire balancer only.
Most vibration in the rear end is caused by tires or driveline angle.
Vibration is a concern with modern, high-mileage tires if they are not "true" both radially and laterally.
They are more susceptible to vibration around the limits of radial and lateral runout of the tire and
wheel assembly. They also require more accurate balancing. Wheel and tire runout checks, truing and
balancing are normally done before axle inspection. Refer to
Driveline Angle
Driveline angularity is the angular relationship between the engine crankshaft (6303), the driveshaft,
and the rear axle pinion. Factors determining driveline angularity include ride height, rear spring, and
engine mounts.
Driveline Angle
Item Description
1 Bottom of the frame
2 Engine crankshaft centerline
3 Engine angle
4 Driveshaft and coupling shaft centerline
5 Driveshaft and coupling shaft angle
6 Rear axle pinion centerline
7 Axle pinion angle
An incorrect driveline (pinion) angle can often be detected by the driving condition in which the
vibration occurs.
A vibration during coastdown from 72 to 56 km/h (45 to 35 mph) is often caused by an
excessive U-joint angle at the axle (pinion nose downward).
A vibration during acceleration, from 56 to 72 km/h (35 to 45 mph) may indicate an excessive U-
joint angle at the axle (pinion nose upward).
Section 204-04 .