Count the upshifts. Is It using all 3 lower gears, and just not using O/D? Is the Overdrive light on?
If 1-3 are shifting normally, and your O/D light isn’t on… make sure the parking brake isn’t stuck and dragging. The extra load will keep it out of OD.
Also check that there isn’t a rodent next in the air filter housing. If the intake is blocked, it won’t’ make as much power, but the shift paints are programmed based on throttle position, so it then it won’t upshift based on the extra throttle command.
