Slave cylinder, at least in my case.
Had a very, very similar problem in my Ranger for quite a while, and it really pissed me off. I have a 5.0EFI with M5OD behind it, and I had big problems with it sometimes completely REFUSING to go into first, typically at stop lights. I would usually roll up to a light from about 10-15 and sit at the light... when it turned green and I went to put it in gear, it completely denied me. After about 5-10 seconds it would usually pop in. Then, between first and second gear, it wouldn't go into second without huge delays. If it wouldn't go into first, it would grind hard going into reverse every time.
After playing with it a while, and having my dad look at (35yr mechanic, hands down the most intelligent car/fabrication person I know) and him being stumped, I thought that just how it was gonna be. We changed the master assembly about 8 months before the slave gave up. That fixed the problem for about a week, and then it was back to square one again.
Finally one day, I started it up, backed out of my driveway, and put it in first... as I took off, it was like I dropped the clutch, and that was the end of that. Slave cylinder toasted.
Typically a problem like this ISN'T the clutch itself (the lined disc that creates the friction to put power to the wheels), since as the lining wears out, you would typically notice the clutch slipping as you accelerate. I would imagine the truck would get easier to put into gear as the clutch itself went out.