Mine our metal, both of them. You can buy replacement plugs at Atlantic British if yours are plastic.
My drain plug has a magnent in it as well.

 

I did some googlin' and found that it is possible for fluid to come out of the axle vent tube(mostly GM's though). I'm going to investigate itthis weekend and clean my engine up in case it's something else. Maybe water got by my nice plastic fill plug into the axle.

 

The vents are for when the diffs heat up and cool down. Expanding and contracting the fluid. If you have water or too much fluid in, it could come out the vent. I would get new plugs and change your fluid asap if you think that is where it is coming from.

 

Normally the evaporation would just leave the vent. It would make sense though that the water evaporates from the axle (maybe taking some oil vapors with it??), cools down when it gets into the venttube, condenses and collects where the tube loops near the front wheel. After a while enough could collect in the tube and spurt out onto the valve cover, exhaust manifold (smoke)and ground when enough pressure builds up. Seems reasonable.

 

If something isn't right, bad fluid or a failing part can heat up or not protect and move the fuid away when it is hot. When you stop, the fluid tries to go back to the hot spot but can't and it comes out the vent. Just a possibility and not why you are leaking. I had to replace an axle on my Expedition because of this. They had relocated the vent tube.

I am just saying you need to open it up and check the fluid before you can tell anything.

 

Yes, you need to pull the fill plug and check the condition of your fluid. If it is bad enough you can fry your axle.
I bought new vent tubes from Atlantic British for $40.

 

Has anybody used "alldata.com"? They have it at my school, it has a ton of good info on any vehicle.
Anyways, there was a TSB about fluid leaking from the breather tube because of a tubeadapter being rotated wrong at the axle. It's a recent TSB (2007) and it's applied to Range Rover's, but still very interesting.