deal alert!!!
Try changing you gear oil in the front diff first, it may be contaminated with water and overfull, or the vent may be plugged causing the gear oil to blow past the seal when it expands.
When all else fails, replace the seal. When you pry the old one out, clean it up real good and you'll find a little series of numbers stamped in the metal. The parts store will use this number to help cross-reference to get the correct part.
Make sure the shaft is clean and free from corrosion where it contacts the seal.