2003 LTD 4WD with P265/65 R17 tires currently. But I've encountered a few resources saying that the P265/70 R17 was either original equipment or an optional size. So what's the deal? Was a P265/70 R17 really factory equipment? If so, I need to rethink my tire choice for an upcoming suspension lift.

Look on the driver door jamb. It will have the oem tire size and pressures. It should be 265/65/17.

Mine had the same size. In my experience though, the 265/70/17 is the better route. With the 265/65/17's my speedo registered about 2.5 MPH fast at 60 MPH (tested with GPS). When I went to 265/70/17's the speedo was dead on.
The 265/70's should fit just fine. On the stock suspension, the first place they will potentially rub is on the front mud flap at full turn. Once the edges of the tires wear a bit, there will be no rub. If you are lifting, there should be no problems.