My truck seems to lack power and isn't shifting/downshifting properly.

What is your question?

I got my truck about 1 month ago. I notice when I get into the higher gears like getting on the freeway the power just isn't there. The truck gears up properly, but when I am coasting and then go to pick up speed,(downshift) it isn't catching, the whole front end shakes really bad. The only way I get it to downshift is if I punch it really hard, I cant just ease into it. I took it to a transmission shop where I was told it wasn't a trans problem. I took it to a repair shop where they diagnosed my check engine light with a bad fuel pump. The problem is still happening though. I am thinking it must be an emissions problem. Maybe a bad knock sensor, catalytic converter, or EGR Valve? I don't know much about vehicles and would greatly appreciate the help.