I'm pretty sure it's the axle seal. Any oil coming from above would have to get past the hot exhaust manifold and it would produce smoke. Smell the fluid and you'll know. Gear oil has a vey distinctive smell.

It's a big job involving the removal of the drive axle. The seal goes into the axle housing, then the axle goes in.

The seal is a common part and should be available in any auto parts store by application... i.e. year, make, model.

It is Timken part #710516. I checked with my parts guy here. Costs less than $20.

You have probably lost some diff oil and that's why you're hearing noise. Be sure to look at the fluid you drain from the diff for metallic glittery stuff, in which case, you've worn the gear faces on the ring and pinion gears. Not the end of everything, but not good either. Let's hope that's not the case.

Good luck!