1998 F150 with 8.8" rear end. I changed the diff. fluid about 6 months ago, used 75w140 as called for and went on my way. When I woke up this morning it was very cold and there was a small stain of gear oil on the driveway from the vent tube of the axle. Drove to work and it is now obvious rear gear oil is leaking out of the tube. I only filled to the bottom of the fill hole when I changed it 6 months ago. Why would gear lube just now be leaking out the vent? I have not been in water or mud since then...or atleast enough to cover the axle. Your help is appreciated.