Rough shifting when going uphill?

When driving uphill the truck lacks power and downshifts only when pressing gas pedal completely but still doesn't have a lot of power and when it downshifts, it is very high rim, very abrupt and rough. As soon as you let off the gas, it loses power and momentum. When on normal flat terrain, it still feels like it's holding back and lacking power. The truck was like this when it was recently bought. Since we bought it, we have replaced the spark plugs, coils, and plugs on coil, throttle body, mass air sensor, and added Lucas to transmission, but the issue is still there. It improved the situation slightly but didn't fix it. Any ideas or suggestions on what direction we need to take to get the issue resolved? Also want to add, there is a kind of a putting or rapping type of noise we can hear coming from, it sounds like, from the rear passenger side of the truck.