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Do you have a 2 or 4 WD truck?
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The link below shows the transmission case and the filler plug is high lighted. The transmission use 80w90 gear oil. Fill it till it starts to run out the hole. You need a suction gun or use a hose on the end of the bottle of oil and squeeze it into the hole. If the trans is grinding and making noise you may need an input shaft bearing. These are notorious for going bad.