This could be one of 4 options:
Bad Master cylinder,
Bad slave cylinder
bad clutch,
or just air in the hydrolic line.
Hard to tell exactly what it is without seeing it, of course, but a quick way to check it out is to check the fluid level under the hood. The clutch mast cylinder sits right next to the brake master cylinder. Pop the top of the clutch master cylinder and make sure there is fluid in it. If the paint on the fire wall is bubbled then you know that you have a bad master cylinder.
If that checks out ok, then you may need to bleed out the slave cylinder that is located between the transmission and engine either left or right side, (prob need a metric 8 , 10 or 12 to brake bleeder loose). You will need one person in the truck to pump up the clutch in reps of 10 pumps and holding it to the floor till the bleeder is opened and then closed. You probably know the drill.
Good luck pal. PM me if you you have any specific questions after working through these steps.