SCORE Derek! Those bring an easy $5K more where I live. Looks great and perfectly normal. That's probably one of the differential or transfer case vent tubes.
Keep clean fluid in that truck and it will last a long time, change it all, from diff to diff. These trucks are much more capable off-road than folks give them credit for.
Get a Tech-2 so you can work on it properly. BlackBear tune will greatly improve driveability and the operation of the transmission, my favorite part.
Run the transfer case through its paces often to keep it ready when you need it. We see posts all the time of some guy who is stuck in 4LO because he never put it in 4LO before.
See our posts on fluid part numbers and capacities using the search function. How's the interior? 2-Tone?
As far as I could tell the seller only listed it on OfferUp, which is not the best platform for selling a vehicle. He said he got a few calls per week but apparently I was the only to show up with cash. And yeah I agree, he underpriced it (or under marketed it). Similar year and mileage were going for $13-16k+ (even without the Z71 pack) and many of those had multiple owners.
Aside from the headliner, the interior is all black (I believe vinyl seats) with 2nd row captains and a full set of Weathertech mats.
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Welcome and Congrats.
The factory coating on the frame is the black stuff you mentioned.
You definitely have a rear main seal leak. Keep an eye on oil level and it will tell you how bad it is.
Good to know about the factory coating. There seemed too much of it to be from a leak but I also haven't encountered undercoating that scrapes off so easily.