I live in alaska and my b4000(ranger) is having clutch problems. the other morning it was about -30f and I started the truck. when it had warmed up for about 20 minutes i went to leave but half of the clutch throw was free play. the bottom half was not enough to allow me to shift into gear with the truck running. I could shift into gear with the truck turned off, but as soon as I started the truck with it in gear it would start to creep with the clutch all the way to the floor. I checked my master cylinder reservoir and it was low so i added fluid to the correct level but the clutch problem still wasn't resolved

 

You didn't say what year it is, but it sounds like you need a new slave cyclinder. To change it you have to pull the tranny, so you should do the clutch while you're at it. I just had to do the one on my jeep and it is a pain. But I found a kit on ebay, and I've seen them for rangers on there too. I think I gave about $130 or so for mine. It might help if you bleed it few times. You may just have a little air in the line. I kept my jeep going for over a year by bleeding it before I finally broke down and changed it.

Hope this helps.