I'm gonna guess that your '01 truck has a locking TC in top gear, and your drivetrain is simulating a manual trans when you go to top gear (probably an OD gear): i.e., the locked TC won't allow the engine to rev to a better place on the torque curve. You 'fall off the torque curve' a bit, and coupled with the top gear's gearing, it is 'flat' in that 45-65 mph range. (And that is with a 5.2 Magnum engine that has a static CR that is 0.5 to 0.75 points higher than the '83 LA 318 engine and a roller cam with a profile that I'll bet is much more like the shorter duration Voodoo's or XE's than the Summit 6900.)
Just translate that into moving off from a stoplight in 1st gear with too low of a DCR. Do it again in 2nd gear unless you revved it in 1st to 4k RPM, and manually upshifted. To fix, you raise the DCR or put in a higher stall TC. The higher stall TC hurts fuel mileage, adds wear & tear, and you get tired of manually 'optimizing' the shift points in everyday driving. It is a lot easier and nicer to drive when the low DCR is fixed instead, fuel mileage is a good bit better (I computed it out and the $137 for a Voodoo is paid for in around 5k miles), etc., etc., etc.
Sorry to go on so much LOL