I agree w/ vital. Sounds like the motor has no air. open the hood and have sone1 rev it a lil and see if you hear a rush of air.
Also a question is the truck new to you or have you have it a long time. If you just got it, it might be "ragged out" people drive these things like you stole them and make the wear down earlier. I test drove one that had 150k and took it home for a weekend my mech. told me that this truck had been abused and need alot of work. It runs and sounds great but really slugish. sounds kinda like your situation