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.