After returning from a 600 mile trip where I was towing about 6,000lbs (including the trailer) I noticed an oil leak.

The vehicle is a 2003 6.0L PSD 4X4.

The oil appears to be coming from the rear of the motor, high enough to leak down both sides of the transmission.
However, it appears to be concentrated more heavily on the drivers side.
My initial thought was the "Rear Main Seal", but I am not really sure.

The oil level seems to be within tolerance (not much has been lost).
The oil is NOT milky (does not appear to have water in it).
There has been no noticeable change in performance (never missed a beat coming home towing a 6,000lb trailer 600 miles).
There has been no decrease / increase in oil temp.
It appears to drip only when the vehicle is running (it will leave a couple of small puddles of oil when parked or left running idle).

Attached are a few pics that may provide some insight as to what I am talking about.
My apologies for the poor quality, they were taken at night with me laying underneath the truck.

Any input will be appreciated.

Thanks,
Cecil