* Brakes: This is the most common cause.
* Parking brake: Is the parking brake engaged, even partially?
* Rear brake drums/shoes: Worn or damaged brake shoes or drums can cause the brakes to drag or lock up. Glazed or contaminated brake shoes are also possibilities. A seized caliper (if you have rear disc brakes, though this is less likely on an '87 Jimmy) is another culprit. One side might be seized, leading to uneven braking and apparent wheel lockup.
* Brake lines/hoses: A restricted or leaking brake line can lead to uneven brake pressure, potentially causing a wheel to lock.
* Master cylinder: A failing master cylinder might not distribute brake fluid evenly.
* Proportioning valve: This valve regulates brake pressure between the front and rear brakes. A malfunctioning valve could send too much pressure to the rear.
* Drivetrain:
* Rear differential: A problem within the differential, such as a seized bearing or a broken axle, can cause a wheel to lock up. This is usually accompanied by other symptoms like noise or difficulty turning.
* Driveshaft: A binding driveshaft can transfer power unevenly, leading to wheel lockup, especially under acceleration or deceleration.
* Wheel bearings: Seized wheel bearings can restrict wheel rotation.
* Suspension:
* Suspension components: While less likely to cause *complete* wheel lockup, severely damaged suspension components (control arms, etc.) *could* bind and interfere with wheel rotation under certain conditions.
Troubleshooting Steps:
1. Safety First: Do NOT drive the vehicle until you determine the cause of the problem. Locked wheels are extremely dangerous.
2. Parking Brake: Ensure the parking brake is completely disengaged.
3. Visual Inspection: Carefully inspect the rear brakes for obvious signs of damage, wear, or fluid leaks. Look for anything binding or out of the ordinary.
4. Brake Pedal: Pump the brake pedal several times. If the pedal feels spongy or goes all the way to the floor, there might be a brake fluid leak or master cylinder issue.
5. Wheel Rotation: Try to rotate each rear wheel by hand. If a wheel is difficult or impossible to turn, there's a problem with that wheel's braking system or drivetrain.
6. Professional Help: If you're not comfortable working on brakes or the drivetrain, take your Jimmy to a qualified mechanic. Improper brake repair can be extremely dangerous.
Important Note: An '87 GMC Jimmy likely has drum brakes in the rear. Working on drum brakes requires specific tools and knowledge. If you're not experienced, seeking professional help is strongly advised. A misdiagnosed or improperly repaired braking system can have serious consequences.
