Truck shifts hard from 1to2 at low speed, But if I stomp on it it shifts fine? I have had the truck for 4 years now,No one could tell me why this happens.

 

What changes between when you start off easily versus putting the pedal to the metal on start off ?

One of the things is engine vacuum and what is vacuum controlled on the trans ? The trans vacuum modulator !

If not working correctly, it can affect shift quality. Suggest you start there and check the modulator and the spool valve it controls.