Good luck.
I'm going through a similar problem with my 2002 Ranger 4x4. Look back a week or so ago for the details.
I'm told Ford won't even consider warranty on tires if the truck is over a year old or over 20,000 kms (12,000 miles or so). When they do warranty a tire, it is usually prorated based on thread depth.
Mine are worn in much the same way but seems to be on all 4. I had the truck in the shop on Friday and I have to go back on Monday to see the warranty manager so we'll see what happens then.
They tell me it is likely the alignment but I had it done in May when I took the snow tires off. What I really can't figure is why they don't check the alignment at the dealership.
Glen