I drive my truck from and to the trails. And most of the time I just cruise at around 60- truck seems to like it and I've never had any issues.
But several years ago a trail took me longer than I thought and I had to haul butt back home to pick up my kid from school. So I did an hour of highway driving at 70-75 mph. Screaming along at 3500 RPM made it in time. But when I got out of my truck I smelled oil- my 14 bolt rear diff puked its fluid out of the vent hose attached onto the underside of the bed. Differential was very hot. It has never done this again- but I haven't driven that fast. In fact I drove it to my parents a few weeks ago with a load of firewood on the highway. It's 2.5 hours by highway (at 60 mph) to get there.
Why? Was it the aftermarket thick cover?
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Was it the fact that my exhaust dumps before the axle and maybe all that exhaust was heating it up?
Don't know.