Well to start off my truck started turning over slow and barely starting,checked the batteries(had two on it at the time) smaller like you get at walmart. Both showed unstable voltage and said to replace. So I ordered a napa bat-7235 battery its a group 31 big comm. one that came on them stock and went back to 1 battery. Well it started once and I went to town and it would not start. Had to have it towed to my mech.. Got it to start there but wasnt turning over right. To make a long story short either the starter killed the batteries or the other way around. But anyhoo 1 new battery and a new starter and i'm in business again. But about 400 bucks lighter!!!! cant complain though trucks been in my family since 2003 and I know the starter hasn't been changed since then if ever. The alternator lasted til last year so pretty good I say! just thought I would share with y'all. These trucks are really AMAZING

 

I have about 240,000 on my '03 dually and just last month lost the final original battery to old age. The truck was starting slower and slower over the course of two-to three weeks and finally one day barely cranked over. Thought the same thing as you did, ended up buying a starter off ebay and it cranks on the first turn over now. WOW, what a difference a new starter makes!
Bought the $40 rebuild kit that has the brush plate and then bought the other misc items like stud and extra large contact plates... now I have a backup Denso starter OEM that will make a welcome addition to my spare parts bag I carry in the tool box.

 

These trucks are one of the few vehicles that have heavy enough starter wiring to actually blow internal connectors in the battery if the starter is shorted.

Moral of the story. Listen to your starter. If it slows down or growls different at any time, get it replaced immediately. It's such a brute that even dragging it'll usually start the engine in warm weather. When you add in all the strain cold does, something gets smoked.