Clutch Wear
Cheston,
Let's assume that you have the original clutch system in your truck.
At each clutch engagement and even on downshifting, a little bit of friction material gets scuffed/worn away, just like brakes. Brakes have a manual clamping system, your leg/master cylinder/caliper create the pressure to clamp the pads to the rotors. When a clutch cover is bolted up, the energy from clamping it down against the diaphragm spring is stored in the spring and pushes back against the disc, creating a controllable clamping system.
Clutch Disc Wear.
As the disc wears and gets thinner, the amount of clampload applied to the disc changes, first at initial wear the clampload actually increases, when the disc is about 1/2 used up, the clampload is at its maximum, then it starts to go down in clampload as the disc continues to wear. When the disc is near it's wornout point, the clampload is roughly equal to the original clampload, but once you get into the lower range you do not have the capacity that you started with.
The closest that I can compare this to is the old high school bell curve.
If you have more power that stock, the loss of clampload will show up sooner as slipping.
NOTE: This does not assume any installation or enviromental conditions, like contamination.