I've never had a starting problem with the truck until recently (always fires on less than 1-rev). A month ago when it was 5 to 10 below zero, the truck would fire once but not start. After several attempts I needed a jump. In the process I discovered that holding the pedal down would make it start even on the coldest day.

This happened a few times under extreme cold weather, and each time full-throttle easily got it started. (Not good reving the engine at 10 below.)

Today it happened at over 40 degrees, and required full-throttle to start.

To me it was though the engine was not getting enough fuel at startup, so I replaced the fuel pressure regulator a month ago.

Once the engine starts, it runs fine at normal RPM for this climate. The plugs are new from this summer and all fluids were changed too.
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I don't know if this is related, so don't focus too much on this, but 20,000 miles ago (with 100,000 current mileage) I noticed the engine would idle rough after driving 20 miles (at a stop light near home). I don't notice this any other time. Just before this stoplight, I am driving on the freeway at 70 or better, but I don't notice it if I'm stuck in traffic minutes before.

I has dawned on me a few minutes ago to replace the fuel filter, but nonetheless, this doesn't seem to be the most likely cause. I am going to do the fuel filter this weekend anyway, but it just seems like there is something else.
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To me, it seems like there is a vacuum leak somewhere. Is the 4.6 T prone to vacuum leaks at any particular point?

 

This sounds like an IAC problem to me. On the 4.6 they are very easy to change. You can try removing it and cleaning it with throttle body cleaner and then reinstalling it.