I have a 2004 with 180K+ miles and have the same problem but only on the passenger front door. The rust is on the bottom part of the door near the seam where the outer shell is welded to the inner door shell.
I've asked around about this issue and the responses I have received are;
1) This problem is very common with Yukon/Tahoe.
I've discussed getting the door sanded and repainted and was basically told it wasn't worth it since the rust more than likely started on the inside and would reappear within a couple of years anyway.
2) This response from another friend seems more common sense to me. I live in the Midwest (Michigan) where they use various types of salt in the winter to clear the roads. These doors have a very good seal when they are closed. When driving in the slushy roads the salt gets up into the door through the seam. Regardless of washes, if the door is not used often (opening and closing) the salt remains in the seam and begins to corrode. The other doors without evidence of rust is because every time you slam the door shut, you are basically shaking the salt of the door.
I say this one makes sense because the one door that is has rust is the door that is used the least (very rarely). So now I randomly walk around, open and close the door just to be safe.
Of course these could all be wrong since I'm not a metalist/chemist or what ever. Maybe that one door just came from a different contractor that either used sub quality metal or improper welding.
My two cents at least.
