Help diagnose a fluid leak (pics)

So a friend of mine sent me these pics from the rear end of a 94 S10. What do you think is causing the caking on the back of the backing plate? He was going the replace the axle seals, but they don’t look too bad in the pics. Brake fluid leak? I am heading over there today to help him out.

http://i.imgur.com/LezTT.jpg

http://i.imgur.com/nOD9h.jpg

Caked side

Clean side

Axle seal , if it was brake fluid the buildup would not be there cause brake fluid washes away with water easily . So any rain or the sorts would rinse shit off , where as a axle seal leaking won’t be easily washed off “leaving a sticky residue” and causing the buidup ya see .

like Jon said axle seal. Also check the vent like make sure it’s clear on something that old. If the vent gets plugged pressure builds in the diff and has to go somewhere.