The gear oil inside of the gearbox keeps the gears lubricated and cool. The gear oil binds to the metal gears and prevents heat friction from metal to metal contact. Once that the gear oil starts to break down, it cannot protect the gears as it should and the gear oil will need to be replaced. You can check and change the gear oil yourself with just a few basic tools.
Jack the front of the vehicle up and put the jack stands securely under the frame of the vehicle on both sides. Make sure that the jack stands are securely and evenly underneath the vehicles frame rail. Lower the vehicle onto the jack stands.
Crawl under the car and locate the gearbox. The gearbox will have a shaft coming out of the centre of the gearbox. There are two plugs on the front of all gearboxes. The lower plug near the bottom of the gearbox is the drain plug. The plug that is higher up on the gearbox is the fill plug. Most drain and fill plugs have a 3/8 square hole in the centre or either have regular bolt heads.
Loosen and remove the top fill plug on the gearbox with either a 3/8 ratchet or a regular closed end wrench. Stick your finger inside of the fill hole. The gearbox oil should be directly below the fill hole to be considered full on any gearbox. If the oil level is lower than the bottom of the fill hole, you will need to add more gear oil until it runs out of the fill hole.
Screw the fill plug back in and tighten it down very tight. Jack the vehicle up and remove the jack stands. Lower the vehicle back to the ground.
You can add gear oil to the gearbox by using a funnel and a long plastic hose.
Make sure that the jack stands are very secure on the frame before crawling under the car.