Could it be the clutch
My car has 200000 miles.
My car has a manual transmission.
This is a symptom of worn or damaged gears within the transmission. It can also be a mis-adjusted shifter linkage, not allowing the gear to fully engage. If your clutch is going bad, you will notice the engine accelerate faster than normal while still moving the truck forward. I feel like you are spinning your tires. You may refer to the factory service manual for adjustment specifications, or have a certified technician, like one from YourMechanic, inspect your slipping issue and verify the correct shifter adjustment.
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