I just saw the pics, I had the same exact leak you have and I also thought it was the rear main seal or my tranny leaking, but it wasn't, my mech put the truck on the lift and he shined the light underneath the valve covers, sure enough both valve cover gaskets were leaking and one of the crankvent hoses that sit on top of the eng. had a hole on it too causing oil to slide down right onto that same spot you showed on the pic.
If u can slide under the truck and shine the light and look up right under the cover gaskets and ck it out.
Keep us posted.