’99 F150 – Rear differential fluid change

So while driving the truck I get a random tug. It is a strange feeling like something in the drivetrain is binding. I owned a Nissan Pathfinder back in the 90s that had the same issue, on the Pathfinder it was the rear differential. I had changed the fluid, but not added friction modifiers for the limited slip clutches. I thought that was the problem with the 99 Ford so I proceeded to change the rear differential fluid. Well after I removed the rear cover I knew this was not the problem, this truck does not even have a limited slip differential, just an old school open differential. I should have looked at the tag on the rear cover, it was stamped ‘3 55’, not ‘3L55’. The L means limited slip, the 3 55 mean it has a 3.55:1 gear ratio. So my grabbing issue is somewhere else. Ford calls for 75W140 in the rear, but I have read 75W140 has issues collecting water, so 75W90 should be fine. Mobil has a 75W90 product that already has the friction modifiers in the oil for limited slip. I know this is not limited slip, but I have already purchased the oil. It will be fine in this differential, you just cannot go the other direction and use non-LS oil in a limited slip differential. I painted the cover red before the reinstall, I just think it looks better than rust.

Update: The grabbing issue is a loose rod in the engine. 🙁 I have pulled too many really heavy loads with this poor Ford.

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