I have a 2001 F250 2wd 7.3 diesel. The transmission is starting to slip. I notice mostly when in high gear if I accelerate to pass. Is there an adjustment? Do I have to replace the entire clutch assembly or just part of it? Could it be something other than the clutch?

Maybe low on fluid, if Automatic.
Maybe low on Fluid in Slave if Hydraulic Clutch.