The inner seals usually leak due to damage (scuffs / scratches) - have you removed the axles recently?
A plugged vent is still another possibility though.

 

Nope never touched them. But it happened very shortly after I changed the fluid. Could I have bumped something?

 

Naah the inner seals are in the axle tube - hard to do much to them when changing the fluid.
Have you used 4x4 much since?
One of ours leaked a little (just enough so the seal/knuckle was damp w oil not dripping though) after a fluid change thought I might have over-filled it so we checked the level and it was fine but then we drove it in 4x4 a couple of times and it hasn't leaked since -we figure maybe draining the fluid and rotating the pinion & spider gears for inspection w/o full oil could have introduced something into the seal or disturbed it and driving it drove/worked it out.

 

I have used 4x4 quite a few times.

 

Got any play in the stub shafts?
Shouldn't move around much.