* U-joints (Universal Joints): These connect the driveshaft to the transmission and rear axle. Worn or damaged U-joints are a very common cause of shaking, especially under load (when in gear). They'll often exhibit a rhythmic clunk or vibration.
* Driveshaft: A bent or unbalanced driveshaft can cause vibrations. This is less likely than bad U-joints, but possible.
* Transmission Problems: While less common if it's only shaking under load, problems *within* the transmission (worn bearings, internal damage) can manifest as vibrations. This is more likely if the shaking is accompanied by other transmission symptoms like slipping, hard shifting, or noises.
* Rear Axle/Differential: Problems in the rear differential, such as worn bearings, damaged gears, or a pinion bearing issue, can transmit vibrations up through the driveshaft.
* Engine Mounts: While unlikely to cause shaking *only* in gear, severely worn engine mounts could contribute to vibrations transferring to the drivetrain under load.
* Tires/Wheels: While less likely, an unbalanced wheel or tire could contribute, but usually you'd feel this vibration regardless of gear. A warped rotor could also cause a vibration, but is more likely felt during braking.
Troubleshooting Steps:
1. Check the U-joints first: This is the easiest and most common cause. Inspect them visually for excessive play or damage. You should be able to grab the driveshaft near the U-joint and feel for movement. Significant play indicates a worn joint.
2. Inspect the driveshaft: Look for any bends or damage.
3. Listen carefully: Pay close attention to the location and type of noise. A rhythmic clunk suggests U-joints, a grinding noise might point to the differential, and a whining sound might indicate transmission issues.
4. Professional Inspection: If you're not comfortable inspecting the drivetrain yourself, take the car to a mechanic specializing in older vehicles. They can properly diagnose the problem and recommend the necessary repairs.
It's important to diagnose the problem accurately before proceeding with repairs. Driving with a damaged drivetrain component could cause further damage.
