I have a 98 F-150 that is leaking from the rear differential vent tube? Is this normal? What causes this and how do I correct it?

 

do you have a hose running from the vent tube or is it just the fitting on the axle housing?

if it is just the fitting then you can go get some rubber hose, that's approved for oil/gas, and hang it up high. there should be some sort of crossmember right above the axle.

 

There is a hose running from the fitting on the axle housing