Truck's transmission grinds whenever I shift to 3rd gear.
This is a very common problem. There is a problem with the synchronizer for 3rd gear inside your transmission. The synchronizer helps the gears mesh properly when changing gears. When a synchronizer goes bad the gears cannot mesh correctly so they grind before going into gear. I would ask for help from a certified transmission mechanic to disassemble your transmission and replace the bad synchronizer for third gear so your transmission won't grind anymore.
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