This is the only rust I can see on the inside/ underside of the door. What's the best way to fix this? should I peel back the door skin and just fab up a patch? It's not rusted thru the door skin at all.

 

If the door skin is solid, thats what I would do. Actually given it's spot it should be an easy fix (as far as rust goes). It might go a little farther than it looks (but still pleanty of room till it starts to curve) so it be a good idea to clean it up before cutting.

 

If you have that sort of rot at the corner then it is more than likely that there is more to be found along the inner lower door seam as well. Dirt falls down the window channel and collects along that seam...any moisture is held by the dirt and soon the rot begins. Ford didn't spend a lot of time or money on undercoating so check the entire seam before you just patch the obvious. Some of the local shops won't even touch these jobs unless the customer agrees to a complete lower door skin patch, too many customers coming back in a year or so complaining about new rust coming through. Good luck...