pinion seal???

I was about to go do some night wheeling with a friend lastnight, and hadnt even hit the first mud hole when something went wrong. I put it in 4wd and backed out the way for my friend to go first. But when i backed up i heard and felt this clunking sound. Got out didnt have a light and couldnt see wut it was. Took it out of 4wd and went forward and it did it again. Then again a few hundred yards down the road it did it again when i punched the gas a little. I got home and i noticed oil or something leaking from the pinion in the front Axel. Does anyone know what could be wrong? and why it made the clunking sound??