Look under your truck. These ol gals are notorious for leaking off the back of the engine. Look at the bottom of the bell housing. See if you have the famous 7.3 drip off of it. If its been wet the water "could" mix with the drip and make the chocolate milkshake looking stain.

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Looks like oily mud/water

 

at 328k can garuntee some fuel line sleeves and some o rings on the fuel bowl are leaking diesel. That diesel rolls down towards the back mixing with whatever oil and drip down the back of the engine. bunch of people always think its a rear main leak 99.99999% its not. its turbo pedestal o rings, hpop lines, and or leaky fuel bowl o rings most commonly fuel bowl drain valve and fuel pressure regulator.


spray some simple green and water and let it soak for 20 min then do it again a couple of times and stuff some old white t shirts around the fuel bowl, hpop lines etc. start the truck and you'll see which shirt gets wet faster etc. im betting combo of oil and fuel from fuel bowl o ring etc.