I'm new to this forum thing, so I'm still learning how it works. I own a 97 f150s 4x4 5 speed. My clutch pedal became firm yesterday out of the blue, as well as in 3rd-5th gears when I shift and then accelerate with foot off the clutch it acts like it's slipping and then it slowly catches and goes. If someone could shed some light for me would be greatly appreciated.
Thanks.

 

Maybe a dumb question, but have you checked the fluid in the clutch master cylinder? I would start with a few simple inspections. First, with the pickup warmed up, put it in 5th gear. Let the clutch out slowly with the engine at idle. if the engine dies right away your clutch discs are decent enough to work. If it slips or shudders before it dies the clutch is slipping. You can also do this in 3rd gear.
I would check everything starting at the pedal. Make sure there is nothing impeding the pedal travel, make sure the firewall is not flexing. Then open the hood and check things out. Put it in first gear and ease the clutch out, watch the engine. Make sure it doesn't lift more than usual. Then crawl under it and check out the engine mounts. I have seen problems that felt like clutch problems actually be a broken motor mount or rear transmission mount.
If you have fluid in your master cylinder, all mounts are good, and everything is good between the pedal and the cylinder and the transmission then it is likely you have a worn out clutch.
I would look for oil leaking from the slave cylinder and maybe splashing onto the clutch. Also, I don't know if these trucks are the same, but I have seen rear main oil seals leak and contaminate the clutch discs with oil.