What year, which motor, which transmission?
There are a few differences between the AX15 behind a 1987 2.2 and the 545RFE in the later trucks ...
But a quick thing - presuming an automatic - How is the fluid level? What kind of shape is it in? If an older truck, when's the last time filter/fluid/bands were taken care of?
What shape is your TPS (Throttle Position Sensor) in? How about the kickdown linkage if it's a A998/A500/A518/42RH/44RH/46RH/42RE/44RE/46RE?
RwP