Black soot near tailpipe

I just returned from a 4000km trip to Nova Scotia and back to Ontario, I was pulling a 5000lb trailer & and the Tacoma pulled like a champ. I have noticed black soot near my tailpipe (all over the rear quarter panel) for a few months now not just while pulling, any ideas????
Next question, I had my Tacos oil and rear diff oil changed at a dealer in Nova Scotia while I was out there, the kid that changed the oils looked like it might have been his first rear diff , I watched him like a hawk checked and made sure he put in the LSD oil in.
Well on return to Ontario I had some strange noises coming from the rear end, so I stopped and checked the diff and it was hot as hell (I think I left finger prints). I also re-greased the hitch; I heard it a few more times but am unsure if it’s the rear diff or trailer (sway-bars or WD hitch)
I had the worst gas mileage I have ever had, 300km per fill up max on the trip.

I have a service appointment at Toyota next week to get the 6-speed manual transmission oil changed and re-check the rear diff.
Is there anything you guys think I should get checked out to ensure the longevity of my Taco???

 

I drove under the speed limit the whole trip, 90 km per hour (50 Mph).
I went out on to the shop floor and saw the LSD oil in bottles, the charged me for three shots?

 

I just noticed you cold air intake and such question, only a K&N air filter.
How hot should the rear diff get?
Just saw your link, They used Toyota LSD diff oil, in 1 L/ quarts jugs

 

Thanks alot for the answers

 

I got the same thing going on, don't think its that big of a deal.........I hope.

 

I've been getting black soot on mine every since I bought it with 3800 miles on it-now has 9700. These trucks are way rich.

 

Looks more like road grime than soot to me.

 

If it's like mine, it's only on the PS by the tailpipe, so it must be soot.

 

Yeah it could be!!! or maybe a dog keeps shitting on my truck at night, but only above my Tail pipe.