I'd say it is from a leaky axle seal. The axle seal if it leaks will then fling itself into the inside of the disc brake- then will come out the vent holes (behind the wheel lugs) and make a big mess.
I'd also check your master cylinder and see if your brake fluid is topped off...
They are pretty easy to change. But you do need a special socket for a nut inside the hub.
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There should be a metal tab locking that nut in place. Pry up with pick/screwdriver then use castle nut socket.
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When I've replaced mine before I put the jack under the axle near the tire and lift up just the side of the truck quite a bit. Then use a jack stand. Then I eat lunch or take a break. This allows the gear oil in the axle tube to drain back to the differential a bit. Otherwise when you remove the axle shaft it will sit there and drip out and make a huge mess.