Question: If the truck is hitting redline but not going anywhere, are you sure the issue is engine related and not drivetrain? Could your clutch/pressure plate/flywheel need some attention?
I'm only guessing but I think if the truck was struggling to put out power it would be hard to reach redline. Further, redline is redline and is tied to the gearing for you to know what your speed will be. <---confusing babble meaning, regardless of how much power my truck spits, 6000 rpm in x gear is gonna give me a pretty much the same speed every time. Difference being how long it takes to get there.
Just an uneducated guess tho