What makes rear axle fluid stink so bad?

My 2007 Frontier is in the shop for rear brakes and axle seals (too damn cold in my garage).  I dropped it off yesterday morning and at 4:00 is wasn't quite ready, the right side was giving the Tech a hard time.  No worries, it's snowing with 9 more inches coming so I'll get it tomorrow morning.

Trish dropped me off on the way to work but it was still on the lift.  Rats.  The counter guy says "Walk in and tell me what you smell".  Man, propane and bad milk?  What is that?  The fluid out of the rear axle.  Truck has 80k miles, lets assume it's factory issue.

I've never smelt anything like that coming out of any car, factory spec stuff?  It specifies 75-90w synthetic.

Thoughts?

Usually sulphr content. Plus, since its a non-circulating, splash lubrication it overheats and gets a burnt smell to go along with the sulphur smell. Plus dirt. Honestly, its one of those smells thatmakes me involuntarily gag.

Sulfur. Acts as an anti-scuff surface between the gears by forming sulfite. Or something like that. Potions was never my thing.

Try European power steering fluid. Different, but just as offensive in its own charming fashion.

Gear oil smell... yeah... I had a T-10 tip over in the back of my Subie wagon years ago.  In the middle of Summer.  In a car without A/C.  I had to rip out and toss the carpet.

Sulfur in the limited slip additive is what stinks. We call it skunk oil.