Truck not getting enough power ?

Any applicable trouble codes? Have you check fuel pressure? I looked at info for 3.9L engine, the fuel pressure shows to be 44--54 psi, so if it is in the middle, it should be fine. You can bring up your fuel pressure, then turn the key off, the fuel pressure should hold for a bit. If it drops like a rock, you need to find the problem. I'd want to see if system is running rich or lean? You may need to check cylinder compression. My info says, 100 psi is min. The lowest reading should be within 70 percent of the highest reading. If the cylinder compression is very bad, it may never run correctly? As far as replacing parts, I usually try to check wiring circuits, first.