One possibility is that the differential is low on fluid. This can cause the gears inside the differential to grind together, which will make a noise. You can check the fluid level by removing the differential cover and looking inside. If the fluid is low, you can add more until it reaches the specified level.
Another possibility is that the wheel bearings are worn out. When this happens, the wheels can move around too much, which can put stress on the differential and cause it to make noise. You can check the wheel bearings by jacking up the rear of the truck and trying to move the wheels back and forth. If there is any play in the wheels, the bearings may need to be replaced.
Finally, it is also possible that the driveshaft is damaged. This can cause the rear end to vibrate, which can make a noise. You can check the driveshaft by looking for any signs of damage, such as dents, cracks, or rust. If the driveshaft is damaged, it will need to be replaced.
If you are not comfortable performing these checks yourself, you can take your truck to a mechanic to have them diagnose the problem.
