It will be one or both of two things. As nicinnz said, it can leak from the welch plug in behind the U Jiont, or its the seal on the transfer case output shaft. I fixed a similar leak in my truck by dismantling the U joint, removing the welch plug (smacked it out down the hollow drive tube). Then ...

Are you sure the lower (front) bushes are shot ? It is the upper bushes that normally tend to wear and fail. I would do the upper first. If it is the upper bushes then Nolathane make a set of replacement bushes. m I have used these on my truck. You need to drill out or burn out the factory bushes ...