I think I might have a blown rear oil seal. But also there is a noticable "clunk" sound when I drive it at low speeds. How hard is that seal to replace? Here's a photo..

 

PITA

I've had one leaking for months now, waiting for some stuff to arrive so I can swap in my e-locker modified axle. There's two seals (inner & outer), the bearing, and the retaining collar that you'll need. You'll also most likely need a press to get the bearing off so you can replace the seal. Can't say what your clunk sound is, but the bearing could be toast in turn destroying the seal.

 

Clunk sound is your brakes catching and letting go because of the oil in the drums