Just purchased a wrangler that was a little neglected to say the least. My rear hinges required my body weight to swing it open. Really. Took spare off, same effort required.
We use a product at work called "Rost Off", sort of like PB Blaster but on steroids. Here's a link: Wurth Rost Off Extra
We have a rep bring it by so you might be able to find it cheaper locally or elsewhere.
Anyway, I sprayed it on top and in between the hinges copiously for several days. After about three days I started to see gunk come out. Also noticed the hinges getting a little easier...Few more days, more gunk and the tail gate now swings with my finger! I couldn't believe it. I thought I was going to have to drill a hole like this guy did: http://www.wranglerforum.com/f5/chea...fix-99045.html
So that was a really long way of saying, try soaking it with your favorite product. I would have never thought with how tight the pins are in the hinges there would not be enough room to get product in, let alone gunk out...but it worked. If not, I was going to try the hole technique mentioned above. Good luck!