Use the the GL-5, I work at grease monkey and i looked your trucks year up in the lube manual and it calls for GL-5 75W-90 for all four boxes. GL-5 is a updated version of GL-4 anway and offers better lubrication and breakdown qualities. Manufacture rec. on changing your fluid is every 15,000 on front and rear and every 30,000 on trans and t-case. With that synthetic i would go 35,000 plus on all
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