Well I will tell you what happened. My Ranger has 50,000. I decided to change the rear differential gear lube. I could not believe how dirty it was. I then decided to change the Transfer Case fluid. (The dealer told me I would probably never have to change it). Well this fluid was so dirty it looked like Motor Oil that had been in the engine for 10,000 miles and it was not red anymore. After that I decided to not wait for the 100K miles to change the plugs. My mileage has dropped a bit and occasionally have a hesitation when starting.
Noble - your right on the mark! Ford states "up to 100K on plugs" in normal driving. Normal, in this day and age is 100% highway driving, with no idling, no towing, no heavy payloads, etc. Severe service is the only schedule to go with, even if you might think you don't need it.
You've found out that a couple quarts of transfer case (ATF) fluid, staying in the same place for 50K is a LONG time. I replace my transfer case fluid every 25K. It's only two quarts (in my transfer case), and I run full synthetic. Plugs, if not removed after 60-70K and given an inspection, along with applying more anti-seize before reinstalling, can lead to plug fouling if you leave them in there 100K or longer. The anti-seize won't last forever, and you might have to drill one of two plugs out as they can easily seize after so many years and so many miles.
If you have a limited slip axle in the rear, then you HAVE to change the fluid every 50K, even for synthetic as the clutch packs require fresh fluid. An open differential running synthetic fluid can easily last 250K before a change is required. I believe 250K is a long time to go between fluid changes, even for an open diff with no clutch packs. It's good to clean the magnet of any metal particles and visually inspect the gears at least every 50K.
Mag Hytec has some SWEET aftermarket differential and transmission covers that not only increase fluid capacity, but offer a fill, and a drain hole with a magnetic plug so you don't have to remove the cover to change the fluid (or use an extraction hose). The Mag Hytecs also include a rubber gasket, so you don't have to mess around with RTV sealant. Just something to think about for your next fluid change.
Dealers know that regular maintenance is key and can bring added sales. However, some dealers choose to say that your oil will last forever, so when your differential, transfercase, transmission or engine seizes up or falls apart, that they'll make $$$ fixing it for you. A little maintenance can go a long way!